cachegrind/docs/cg-manual.xml

<?xml version="1.0"?> <!-- -*- sgml -*- -->
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<chapter id="cg-manual" xreflabel="Cachegrind: a cache-miss profiler">
<title>Cachegrind: a cache profiler</title>

<para>Detailed technical documentation on how Cachegrind works is
available in <xref linkend="cg-tech-docs"/>.  If you only want to know
how to <command>use</command> it, this is the page you need to
read.</para>


<sect1 id="cg-manual.cache" xreflabel="Cache profiling">
<title>Cache profiling</title>

<para>To use this tool, you must specify
<computeroutput>--tool=cachegrind</computeroutput> on the
Valgrind command line.</para>

<para>Cachegrind is a tool for doing cache simulations and
annotating your source line-by-line with the number of cache
misses.  In particular, it records:</para>
<itemizedlist>
  <listitem>
    <para>L1 instruction cache reads and misses;</para>
  </listitem>
  <listitem>
    <para>L1 data cache reads and read misses, writes and write
    misses;</para>
  </listitem>
  <listitem>
    <para>L2 unified cache reads and read misses, writes and
    writes misses.</para>
  </listitem>
</itemizedlist>

<para>On a modern machine, an L1 miss will typically cost
around 10 cycles, and an L2 miss can cost as much as 200
cycles. Detailed cache profiling can be very useful for improving
the performance of your program.</para>

<para>Also, since one instruction cache read is performed per
instruction executed, you can find out how many instructions are
executed per line, which can be useful for traditional profiling
and test coverage.</para>

<para>Any feedback, bug-fixes, suggestions, etc, welcome.</para>



<sect2 id="cg-manual.overview" xreflabel="Overview">
<title>Overview</title>

<para>First off, as for normal Valgrind use, you probably want to
compile with debugging info (the
<computeroutput>-g</computeroutput> flag).  But by contrast with
normal Valgrind use, you probably <command>do</command> want to turn
optimisation on, since you should profile your program as it will
be normally run.</para>

<para>The two steps are:</para>
<orderedlist>
  <listitem>
    <para>Run your program with <computeroutput>valgrind
    --tool=cachegrind</computeroutput> in front of the normal
    command line invocation.  When the program finishes,
    Cachegrind will print summary cache statistics. It also
    collects line-by-line information in a file
    <computeroutput>cachegrind.out.pid</computeroutput>, where
    <computeroutput>pid</computeroutput> is the program's process
    id.</para>

    <para>This step should be done every time you want to collect
    information about a new program, a changed program, or about
    the same program with different input.</para>
  </listitem>

  <listitem>
    <para>Generate a function-by-function summary, and possibly
    annotate source files, using the supplied
    <computeroutput>cg_annotate</computeroutput> program. Source
    files to annotate can be specified manually, or manually on
    the command line, or "interesting" source files can be
    annotated automatically with the
    <computeroutput>--auto=yes</computeroutput> option.  You can
    annotate C/C++ files or assembly language files equally
    easily.</para>

    <para>This step can be performed as many times as you like
    for each Step 2.  You may want to do multiple annotations
    showing different information each time.</para>
  </listitem>

</orderedlist>

<para>The steps are described in detail in the following
sections.</para>

</sect2>


<sect2 id="cache-sim" xreflabel="Cache simulation specifics">
<title>Cache simulation specifics</title>

<para>Cachegrind uses a simulation for a machine with a split L1
cache and a unified L2 cache.  This configuration is used for all
(modern) x86-based machines we are aware of.  Old Cyrix CPUs had
a unified I and D L1 cache, but they are ancient history
now.</para>

<para>The more specific characteristics of the simulation are as
follows.</para>

<itemizedlist>

  <listitem>
    <para>Write-allocate: when a write miss occurs, the block
    written to is brought into the D1 cache.  Most modern caches
    have this property.</para>
  </listitem>

  <listitem>
    <para>Bit-selection hash function: the line(s) in the cache
    to which a memory block maps is chosen by the middle bits
    M--(M+N-1) of the byte address, where:</para>
    <itemizedlist>
      <listitem>
        <para>line size = 2^M bytes</para>
      </listitem>
      <listitem>
        <para>(cache size / line size) = 2^N bytes</para>
      </listitem>
    </itemizedlist> 
  </listitem>

  <listitem>
    <para>Inclusive L2 cache: the L2 cache replicates all the
    entries of the L1 cache.  This is standard on Pentium chips,
    but AMD Athlons use an exclusive L2 cache that only holds
    blocks evicted from L1.  Ditto AMD Durons and most modern
    VIAs.</para>
  </listitem>

</itemizedlist>

<para>The cache configuration simulated (cache size,
associativity and line size) is determined automagically using
the CPUID instruction.  If you have an old machine that (a)
doesn't support the CPUID instruction, or (b) supports it in an
early incarnation that doesn't give any cache information, then
Cachegrind will fall back to using a default configuration (that
of a model 3/4 Athlon).  Cachegrind will tell you if this
happens.  You can manually specify one, two or all three levels
(I1/D1/L2) of the cache from the command line using the
<computeroutput>--I1</computeroutput>,
<computeroutput>--D1</computeroutput> and
<computeroutput>--L2</computeroutput> options.</para>


<para>Other noteworthy behaviour:</para>

<itemizedlist>
  <listitem>
    <para>References that straddle two cache lines are treated as
    follows:</para>
    <itemizedlist>
      <listitem>
        <para>If both blocks hit --&gt; counted as one hit</para>
      </listitem>
      <listitem>
        <para>If one block hits, the other misses --&gt; counted
        as one miss.</para>
      </listitem>
      <listitem>
        <para>If both blocks miss --&gt; counted as one miss (not
        two)</para>
      </listitem>
    </itemizedlist>
  </listitem>

  <listitem>
    <para>Instructions that modify a memory location
    (eg. <computeroutput>inc</computeroutput> and
    <computeroutput>dec</computeroutput>) are counted as doing
    just a read, ie. a single data reference.  This may seem
    strange, but since the write can never cause a miss (the read
    guarantees the block is in the cache) it's not very
    interesting.</para>

    <para>Thus it measures not the number of times the data cache
    is accessed, but the number of times a data cache miss could
    occur.</para>
  </listitem>

</itemizedlist>

<para>If you are interested in simulating a cache with different
properties, it is not particularly hard to write your own cache
simulator, or to modify the existing ones in
<computeroutput>vg_cachesim_I1.c</computeroutput>,
<computeroutput>vg_cachesim_D1.c</computeroutput>,
<computeroutput>vg_cachesim_L2.c</computeroutput> and
<computeroutput>vg_cachesim_gen.c</computeroutput>.  We'd be
interested to hear from anyone who does.</para>

</sect2>

</sect1>



<sect1 id="cg-manual.profile" xreflabel="Profiling programs">
<title>Profiling programs</title>

<para>To gather cache profiling information about the program
<computeroutput>ls -l</computeroutput>, invoke Cachegrind like
this:</para>

<programlisting><![CDATA[
valgrind --tool=cachegrind ls -l]]></programlisting>

<para>The program will execute (slowly).  Upon completion,
summary statistics that look like this will be printed:</para>

<programlisting><![CDATA[
==31751== I   refs:      27,742,716
==31751== I1  misses:           276
==31751== L2  misses:           275
==31751== I1  miss rate:        0.0%
==31751== L2i miss rate:        0.0%
==31751== 
==31751== D   refs:      15,430,290  (10,955,517 rd + 4,474,773 wr)
==31751== D1  misses:        41,185  (    21,905 rd +    19,280 wr)
==31751== L2  misses:        23,085  (     3,987 rd +    19,098 wr)
==31751== D1  miss rate:        0.2% (       0.1%   +       0.4%)
==31751== L2d miss rate:        0.1% (       0.0%   +       0.4%)
==31751== 
==31751== L2 misses:         23,360  (     4,262 rd +    19,098 wr)
==31751== L2 miss rate:         0.0% (       0.0%   +       0.4%)]]></programlisting>

<para>Cache accesses for instruction fetches are summarised
first, giving the number of fetches made (this is the number of
instructions executed, which can be useful to know in its own
right), the number of I1 misses, and the number of L2 instruction
(<computeroutput>L2i</computeroutput>) misses.</para>

<para>Cache accesses for data follow. The information is similar
to that of the instruction fetches, except that the values are
also shown split between reads and writes (note each row's
<computeroutput>rd</computeroutput> and
<computeroutput>wr</computeroutput> values add up to the row's
total).</para>

<para>Combined instruction and data figures for the L2 cache
follow that.</para>



<sect2 id="cg-manual.outputfile" xreflabel="Output file">
<title>Output file</title>

<para>As well as printing summary information, Cachegrind also
writes line-by-line cache profiling information to a file named
<computeroutput>cachegrind.out.pid</computeroutput>.  This file
is human-readable, but is best interpreted by the accompanying
program <computeroutput>cg_annotate</computeroutput>, described
in the next section.</para>

<para>Things to note about the
<computeroutput>cachegrind.out.pid</computeroutput>
file:</para>

<itemizedlist>
  <listitem>
    <para>It is written every time Cachegrind is run, and will
    overwrite any existing
    <computeroutput>cachegrind.out.pid</computeroutput>
    in the current directory (but that won't happen very often
    because it takes some time for process ids to be
    recycled).</para>
  </listitem>
  <listitem>
    <para>It can be huge: <computeroutput>ls -l</computeroutput>
    generates a file of about 350KB.  Browsing a few files and
    web pages with a Konqueror built with full debugging
    information generates a file of around 15 MB.</para>
  </listitem>
</itemizedlist>

<para>The <computeroutput>.pid</computeroutput> suffix
on the output file name 
serves
two purposes.  Firstly, it means you don't have to rename old log
files that you don't want to overwrite.  Secondly, and more
importantly, it allows correct profiling with the
<computeroutput>--trace-children=yes</computeroutput> option of
programs that spawn child processes.</para>

</sect2>



<sect2 id="cg-manual.cgopts" xreflabel="Cachegrind options">
<title>Cachegrind options</title>

<para>Cache-simulation specific options are:</para>

<screen><![CDATA[
--I1=<size>,<associativity>,<line_size>
--D1=<size>,<associativity>,<line_size>
--L2=<size>,<associativity>,<line_size>

[default: uses CPUID for automagic cache configuration]]]></screen>

<para>Manually specifies the I1/D1/L2 cache configuration, where
<computeroutput>size</computeroutput> and
<computeroutput>line_size</computeroutput> are measured in bytes.
The three items must be comma-separated, but with no spaces,
eg:</para>

<programlisting><![CDATA[
valgrind --tool=cachegrind --I1=65535,2,64]]></programlisting>

<para>You can specify one, two or three of the I1/D1/L2 caches.
Any level not manually specified will be simulated using the
configuration found in the normal way (via the CPUID instruction,
or failing that, via defaults).</para>

</sect2>


  
<sect2 id="cg-manual.annotate" xreflabel="Annotating C/C++ programs">
<title>Annotating C/C++ programs</title>

<para>Before using <computeroutput>cg_annotate</computeroutput>,
it is worth widening your window to be at least 120-characters
wide if possible, as the output lines can be quite long.</para>

<para>To get a function-by-function summary, run
<computeroutput>cg_annotate --pid</computeroutput> in a directory
containing a <computeroutput>cachegrind.out.pid</computeroutput>
file.  The <emphasis>--pid</emphasis> is required so that
<computeroutput>cg_annotate</computeroutput> knows which log file
to use when several are present.</para>

<para>The output looks like this:</para>

<programlisting><![CDATA[
--------------------------------------------------------------------------------
I1 cache:              65536 B, 64 B, 2-way associative
D1 cache:              65536 B, 64 B, 2-way associative
L2 cache:              262144 B, 64 B, 8-way associative
Command:               concord vg_to_ucode.c
Events recorded:       Ir I1mr I2mr Dr D1mr D2mr Dw D1mw D2mw
Events shown:          Ir I1mr I2mr Dr D1mr D2mr Dw D1mw D2mw
Event sort order:      Ir I1mr I2mr Dr D1mr D2mr Dw D1mw D2mw
Threshold:             99%
Chosen for annotation:
Auto-annotation:       on

--------------------------------------------------------------------------------
Ir         I1mr I2mr Dr         D1mr   D2mr  Dw        D1mw   D2mw
--------------------------------------------------------------------------------
27,742,716  276  275 10,955,517 21,905 3,987 4,474,773 19,280 19,098  PROGRAM TOTALS

--------------------------------------------------------------------------------
Ir        I1mr I2mr Dr        D1mr  D2mr  Dw        D1mw   D2mw    file:function
--------------------------------------------------------------------------------
8,821,482    5    5 2,242,702 1,621    73 1,794,230      0      0  getc.c:_IO_getc
5,222,023    4    4 2,276,334    16    12   875,959      1      1  concord.c:get_word
2,649,248    2    2 1,344,810 7,326 1,385         .      .      .  vg_main.c:strcmp
2,521,927    2    2   591,215     0     0   179,398      0      0  concord.c:hash
2,242,740    2    2 1,046,612   568    22   448,548      0      0  ctype.c:tolower
1,496,937    4    4   630,874 9,000 1,400   279,388      0      0  concord.c:insert
  897,991   51   51   897,831    95    30        62      1      1  ???:???
  598,068    1    1   299,034     0     0   149,517      0      0  ../sysdeps/generic/lockfile.c:__flockfile
  598,068    0    0   299,034     0     0   149,517      0      0  ../sysdeps/generic/lockfile.c:__funlockfile
  598,024    4    4   213,580    35    16   149,506      0      0  vg_clientmalloc.c:malloc
  446,587    1    1   215,973 2,167   430   129,948 14,057 13,957  concord.c:add_existing
  341,760    2    2   128,160     0     0   128,160      0      0  vg_clientmalloc.c:vg_trap_here_WRAPPER
  320,782    4    4   150,711   276     0    56,027     53     53  concord.c:init_hash_table
  298,998    1    1   106,785     0     0    64,071      1      1  concord.c:create
  149,518    0    0   149,516     0     0         1      0      0  ???:tolower@@GLIBC_2.0
  149,518    0    0   149,516     0     0         1      0      0  ???:fgetc@@GLIBC_2.0
   95,983    4    4    38,031     0     0    34,409  3,152  3,150  concord.c:new_word_node
   85,440    0    0    42,720     0     0    21,360      0      0  vg_clientmalloc.c:vg_bogus_epilogue]]></programlisting>


<para>First up is a summary of the annotation options:</para>
                    
<itemizedlist>

  <listitem>
    <para>I1 cache, D1 cache, L2 cache: cache configuration.  So
    you know the configuration with which these results were
    obtained.</para>
  </listitem>

  <listitem>
    <para>Command: the command line invocation of the program
      under examination.</para>
  </listitem>

  <listitem>
   <para>Events recorded: event abbreviations are:</para>
   <itemizedlist>
     <listitem>
       <para><computeroutput>Ir </computeroutput>: I cache reads
       (ie. instructions executed)</para>
     </listitem>
     <listitem>
       <para><computeroutput>I1mr</computeroutput>: I1 cache read
       misses</para>
     </listitem>
     <listitem>
       <para><computeroutput>I2mr</computeroutput>: L2 cache
       instruction read misses</para>
     </listitem>
     <listitem>
       <para><computeroutput>Dr </computeroutput>: D cache reads
       (ie. memory reads)</para>
     </listitem>
     <listitem>
       <para><computeroutput>D1mr</computeroutput>: D1 cache read
       misses</para>
     </listitem>
     <listitem>
       <para><computeroutput>D2mr</computeroutput>: L2 cache data
       read misses</para>
     </listitem>
     <listitem>
       <para><computeroutput>Dw </computeroutput>: D cache writes
       (ie. memory writes)</para>
     </listitem>
     <listitem>
       <para><computeroutput>D1mw</computeroutput>: D1 cache write
       misses</para>
     </listitem>
     <listitem>
       <para><computeroutput>D2mw</computeroutput>: L2 cache data
       write misses</para>
     </listitem>
   </itemizedlist>

   <para>Note that D1 total accesses is given by
   <computeroutput>D1mr</computeroutput> +
   <computeroutput>D1mw</computeroutput>, and that L2 total
   accesses is given by <computeroutput>I2mr</computeroutput> +
   <computeroutput>D2mr</computeroutput> +
   <computeroutput>D2mw</computeroutput>.</para>
 </listitem>

 <listitem>
   <para>Events shown: the events shown (a subset of events
   gathered).  This can be adjusted with the
   <computeroutput>--show</computeroutput> option.</para>
  </listitem>

  <listitem>
    <para>Event sort order: the sort order in which functions are
    shown.  For example, in this case the functions are sorted
    from highest <computeroutput>Ir</computeroutput> counts to
    lowest.  If two functions have identical
    <computeroutput>Ir</computeroutput> counts, they will then be
    sorted by <computeroutput>I1mr</computeroutput> counts, and
    so on.  This order can be adjusted with the
    <computeroutput>--sort</computeroutput> option.</para>

    <para>Note that this dictates the order the functions appear.
    It is <command>not</command> the order in which the columns
    appear; that is dictated by the "events shown" line (and can
    be changed with the <computeroutput>--show</computeroutput>
    option).</para>
  </listitem>

  <listitem>
    <para>Threshold: <computeroutput>cg_annotate</computeroutput>
    by default omits functions that cause very low numbers of
    misses to avoid drowning you in information.  In this case,
    cg_annotate shows summaries the functions that account for
    99% of the <computeroutput>Ir</computeroutput> counts;
    <computeroutput>Ir</computeroutput> is chosen as the
    threshold event since it is the primary sort event.  The
    threshold can be adjusted with the
    <computeroutput>--threshold</computeroutput>
    option.</para>
  </listitem>

  <listitem>
    <para>Chosen for annotation: names of files specified
    manually for annotation; in this case none.</para>
  </listitem>

  <listitem>
    <para>Auto-annotation: whether auto-annotation was requested
    via the <computeroutput>--auto=yes</computeroutput>
    option. In this case no.</para>
  </listitem>

</itemizedlist>

<para>Then follows summary statistics for the whole
program. These are similar to the summary provided when running
<computeroutput>valgrind
--tool=cachegrind</computeroutput>.</para>
  
<para>Then follows function-by-function statistics. Each function
is identified by a
<computeroutput>file_name:function_name</computeroutput> pair. If
a column contains only a dot it means the function never performs
that event (eg. the third row shows that
<computeroutput>strcmp()</computeroutput> contains no
instructions that write to memory). The name
<computeroutput>???</computeroutput> is used if the the file name
and/or function name could not be determined from debugging
information. If most of the entries have the form
<computeroutput>???:???</computeroutput> the program probably
wasn't compiled with <computeroutput>-g</computeroutput>.  If any
code was invalidated (either due to self-modifying code or
unloading of shared objects) its counts are aggregated into a
single cost centre written as
<computeroutput>(discarded):(discarded)</computeroutput>.</para>

<para>It is worth noting that functions will come from three
types of source files:</para>

<orderedlist>
  <listitem>
    <para>From the profiled program
    (<filename>concord.c</filename> in this example).</para>
  </listitem>
  <listitem>
    <para>From libraries (eg. <filename>getc.c</filename>)</para>
  </listitem>
  <listitem>
    <para>From Valgrind's implementation of some libc functions
    (eg. <computeroutput>vg_clientmalloc.c:malloc</computeroutput>).
    These are recognisable because the filename begins with
    <computeroutput>vg_</computeroutput>, and is probably one of
    <filename>vg_main.c</filename>,
    <filename>vg_clientmalloc.c</filename> or
    <filename>vg_mylibc.c</filename>.</para>
  </listitem>

</orderedlist>

<para>There are two ways to annotate source files -- by choosing
them manually, or with the
<computeroutput>--auto=yes</computeroutput> option. To do it
manually, just specify the filenames as arguments to
<computeroutput>cg_annotate</computeroutput>. For example, the
output from running <filename>cg_annotate concord.c</filename>
for our example produces the same output as above followed by an
annotated version of <filename>concord.c</filename>, a section of
which looks like:</para>

<programlisting><![CDATA[
--------------------------------------------------------------------------------
-- User-annotated source: concord.c
--------------------------------------------------------------------------------
Ir        I1mr I2mr Dr      D1mr  D2mr  Dw      D1mw   D2mw

[snip]

        .    .    .       .     .     .       .      .      .  void init_hash_table(char *file_name, Word_Node *table[])
        3    1    1       .     .     .       1      0      0  {
        .    .    .       .     .     .       .      .      .      FILE *file_ptr;
        .    .    .       .     .     .       .      .      .      Word_Info *data;
        1    0    0       .     .     .       1      1      1      int line = 1, i;
        .    .    .       .     .     .       .      .      .
        5    0    0       .     .     .       3      0      0      data = (Word_Info *) create(sizeof(Word_Info));
        .    .    .       .     .     .       .      .      .
    4,991    0    0   1,995     0     0     998      0      0      for (i = 0; i < TABLE_SIZE; i++)
    3,988    1    1   1,994     0     0     997     53     52          table[i] = NULL;
        .    .    .       .     .     .       .      .      .
        .    .    .       .     .     .       .      .      .      /* Open file, check it. */
        6    0    0       1     0     0       4      0      0      file_ptr = fopen(file_name, "r");
        2    0    0       1     0     0       .      .      .      if (!(file_ptr)) {
        .    .    .       .     .     .       .      .      .          fprintf(stderr, "Couldn't open '%s'.\n", file_name);
        1    1    1       .     .     .       .      .      .          exit(EXIT_FAILURE);
        .    .    .       .     .     .       .      .      .      }
        .    .    .       .     .     .       .      .      .
  165,062    1    1  73,360     0     0  91,700      0      0      while ((line = get_word(data, line, file_ptr)) != EOF)
  146,712    0    0  73,356     0     0  73,356      0      0          insert(data->;word, data->line, table);
        .    .    .       .     .     .       .      .      .
        4    0    0       1     0     0       2      0      0      free(data);
        4    0    0       1     0     0       2      0      0      fclose(file_ptr);
        3    0    0       2     0     0       .      .      .  }]]></programlisting>

<para>(Although column widths are automatically minimised, a wide
terminal is clearly useful.)</para>
  
<para>Each source file is clearly marked
(<computeroutput>User-annotated source</computeroutput>) as
having been chosen manually for annotation.  If the file was
found in one of the directories specified with the
<computeroutput>-I / --include</computeroutput> option, the directory
and file are both given.</para>

<para>Each line is annotated with its event counts.  Events not
applicable for a line are represented by a `.'; this is useful
for distinguishing between an event which cannot happen, and one
which can but did not.</para>

<para>Sometimes only a small section of a source file is
executed.  To minimise uninteresting output, Cachegrind only shows
annotated lines and lines within a small distance of annotated
lines.  Gaps are marked with the line numbers so you know which
part of a file the shown code comes from, eg:</para>

<programlisting><![CDATA[
(figures and code for line 704)
-- line 704 ----------------------------------------
-- line 878 ----------------------------------------
(figures and code for line 878)]]></programlisting>

<para>The amount of context to show around annotated lines is
controlled by the <computeroutput>--context</computeroutput>
option.</para>

<para>To get automatic annotation, run
<computeroutput>cg_annotate --auto=yes</computeroutput>.
cg_annotate will automatically annotate every source file it can
find that is mentioned in the function-by-function summary.
Therefore, the files chosen for auto-annotation are affected by
the <computeroutput>--sort</computeroutput> and
<computeroutput>--threshold</computeroutput> options.  Each
source file is clearly marked (<computeroutput>Auto-annotated
source</computeroutput>) as being chosen automatically.  Any
files that could not be found are mentioned at the end of the
output, eg:</para>

<programlisting><![CDATA[
------------------------------------------------------------------
The following files chosen for auto-annotation could not be found:
------------------------------------------------------------------
  getc.c
  ctype.c
  ../sysdeps/generic/lockfile.c]]></programlisting>

<para>This is quite common for library files, since libraries are
usually compiled with debugging information, but the source files
are often not present on a system.  If a file is chosen for
annotation <command>both</command> manually and automatically, it
is marked as <computeroutput>User-annotated
source</computeroutput>. Use the <computeroutput>-I /
--include</computeroutput> option to tell Valgrind where to look
for source files if the filenames found from the debugging
information aren't specific enough.</para>

<para>Beware that cg_annotate can take some time to digest large
<computeroutput>cachegrind.out.pid</computeroutput> files,
e.g. 30 seconds or more.  Also beware that auto-annotation can
produce a lot of output if your program is large!</para>

</sect2>


<sect2 id="cg-manual.assembler" xreflabel="Annotating assembler programs">
<title>Annotating assembler programs</title>

<para>Valgrind can annotate assembler programs too, or annotate
the assembler generated for your C program.  Sometimes this is
useful for understanding what is really happening when an
interesting line of C code is translated into multiple
instructions.</para>

<para>To do this, you just need to assemble your
<computeroutput>.s</computeroutput> files with assembler-level
debug information.  gcc doesn't do this, but you can use the GNU
assembler with the <computeroutput>--gstabs</computeroutput>
option to generate object files with this information, eg:</para>

<programlisting><![CDATA[
as --gstabs foo.s]]></programlisting>

<para>You can then profile and annotate source files in the same
way as for C/C++ programs.</para>

</sect2>

</sect1>


<sect1 id="cg-manual.annopts" xreflabel="cg_annotate options">
<title><computeroutput>cg_annotate</computeroutput> options</title>

<itemizedlist>

  <listitem id="pid">
    <para><computeroutput>--pid</computeroutput></para>
    <para>Indicates which
    <computeroutput>cachegrind.out.pid</computeroutput> file to
    read.  Not actually an option -- it is required.</para>
  </listitem>
    
  <listitem>
    <para><computeroutput>-h, --help</computeroutput></para>
    <para><computeroutput>-v, --version</computeroutput></para>
    <para>Help and version, as usual.</para>
  </listitem>

  <listitem id="sort">
    <para><computeroutput>--sort=A,B,C</computeroutput> [default:
    order in
    <computeroutput>cachegrind.out.pid</computeroutput>]</para>
    <para>Specifies the events upon which the sorting of the
    function-by-function entries will be based.  Useful if you
    want to concentrate on eg. I cache misses
    (<computeroutput>--sort=I1mr,I2mr</computeroutput>), or D
    cache misses
    (<computeroutput>--sort=D1mr,D2mr</computeroutput>), or L2
    misses
    (<computeroutput>--sort=D2mr,I2mr</computeroutput>).</para>
  </listitem>

  <listitem id="show">
    <para><computeroutput>--show=A,B,C</computeroutput> [default:
    all, using order in
    <computeroutput>cachegrind.out.pid</computeroutput>]</para>
    <para>Specifies which events to show (and the column
    order). Default is to use all present in the
    <computeroutput>cachegrind.out.pid</computeroutput> file (and
    use the order in the file).</para>
  </listitem>

  <listitem id="threshold">
    <para><computeroutput>--threshold=X</computeroutput>
    [default: 99%]</para>
    <para>Sets the threshold for the function-by-function
    summary.  Functions are shown that account for more than X%
    of the primary sort event.  If auto-annotating, also affects
    which files are annotated.</para>
      
    <para>Note: thresholds can be set for more than one of the
    events by appending any events for the
    <computeroutput>--sort</computeroutput> option with a colon
    and a number (no spaces, though).  E.g. if you want to see
    the functions that cover 99% of L2 read misses and 99% of L2
    write misses, use this option:</para>
    <para><computeroutput>--sort=D2mr:99,D2mw:99</computeroutput></para>
  </listitem>

  <listitem id="auto">
    <para><computeroutput>--auto=no</computeroutput> [default]</para>
    <para><computeroutput>--auto=yes</computeroutput></para>
    <para>When enabled, automatically annotates every file that
    is mentioned in the function-by-function summary that can be
    found.  Also gives a list of those that couldn't be found.</para>
  </listitem>

  <listitem id="context">
    <para><computeroutput>--context=N</computeroutput> [default:
    8]</para>
    <para>Print N lines of context before and after each
    annotated line.  Avoids printing large sections of source
    files that were not executed.  Use a large number
    (eg. 10,000) to show all source lines.</para>
  </listitem>

  <listitem id="include">
    <para><computeroutput>-I&lt;dir&gt;,
      --include=&lt;dir&gt;</computeroutput> [default: empty
      string]</para>
    <para>Adds a directory to the list in which to search for
    files.  Multiple -I/--include options can be given to add
    multiple directories.</para>
  </listitem>

</itemizedlist>
  


<sect2>
<title>Warnings</title>

<para>There are a couple of situations in which
<computeroutput>cg_annotate</computeroutput> issues
warnings.</para>

<itemizedlist>
  <listitem>
    <para>If a source file is more recent than the
    <computeroutput>cachegrind.out.pid</computeroutput> file.
    This is because the information in
    <computeroutput>cachegrind.out.pid</computeroutput> is only
    recorded with line numbers, so if the line numbers change at
    all in the source (eg.  lines added, deleted, swapped), any
    annotations will be incorrect.</para>
  </listitem>
  <listitem>
    <para>If information is recorded about line numbers past the
    end of a file.  This can be caused by the above problem,
    ie. shortening the source file while using an old
    <computeroutput>cachegrind.out.pid</computeroutput> file.  If
    this happens, the figures for the bogus lines are printed
    anyway (clearly marked as bogus) in case they are
    important.</para>
  </listitem>
</itemizedlist>

</sect2>



<sect2>
<title>Things to watch out for</title>

<para>Some odd things that can occur during annotation:</para>

<itemizedlist>
  <listitem>
    <para>If annotating at the assembler level, you might see
    something like this:</para>
<programlisting><![CDATA[
      1    0    0  .    .    .  .    .    .          leal -12(%ebp),%eax
      1    0    0  .    .    .  1    0    0          movl %eax,84(%ebx)
      2    0    0  0    0    0  1    0    0          movl $1,-20(%ebp)
      .    .    .  .    .    .  .    .    .          .align 4,0x90
      1    0    0  .    .    .  .    .    .          movl $.LnrB,%eax
      1    0    0  .    .    .  1    0    0          movl %eax,-16(%ebp)]]></programlisting>

    <para>How can the third instruction be executed twice when
    the others are executed only once?  As it turns out, it
    isn't.  Here's a dump of the executable, using
    <computeroutput>objdump -d</computeroutput>:</para>
<programlisting><![CDATA[
      8048f25:       8d 45 f4                lea    0xfffffff4(%ebp),%eax
      8048f28:       89 43 54                mov    %eax,0x54(%ebx)
      8048f2b:       c7 45 ec 01 00 00 00    movl   $0x1,0xffffffec(%ebp)
      8048f32:       89 f6                   mov    %esi,%esi
      8048f34:       b8 08 8b 07 08          mov    $0x8078b08,%eax
      8048f39:       89 45 f0                mov    %eax,0xfffffff0(%ebp)]]></programlisting>

    <para>Notice the extra <computeroutput>mov
    %esi,%esi</computeroutput> instruction.  Where did this come
    from?  The GNU assembler inserted it to serve as the two
    bytes of padding needed to align the <computeroutput>movl
    $.LnrB,%eax</computeroutput> instruction on a four-byte
    boundary, but pretended it didn't exist when adding debug
    information.  Thus when Valgrind reads the debug info it
    thinks that the <computeroutput>movl
    $0x1,0xffffffec(%ebp)</computeroutput> instruction covers the
    address range 0x8048f2b--0x804833 by itself, and attributes
    the counts for the <computeroutput>mov
    %esi,%esi</computeroutput> to it.</para>
  </listitem>

  <listitem>
    <para>Inlined functions can cause strange results in the
    function-by-function summary.  If a function
    <computeroutput>inline_me()</computeroutput> is defined in
    <filename>foo.h</filename> and inlined in the functions
    <computeroutput>f1()</computeroutput>,
    <computeroutput>f2()</computeroutput> and
    <computeroutput>f3()</computeroutput> in
    <filename>bar.c</filename>, there will not be a
    <computeroutput>foo.h:inline_me()</computeroutput> function
    entry.  Instead, there will be separate function entries for
    each inlining site, ie.
    <computeroutput>foo.h:f1()</computeroutput>,
    <computeroutput>foo.h:f2()</computeroutput> and
    <computeroutput>foo.h:f3()</computeroutput>.  To find the
    total counts for
    <computeroutput>foo.h:inline_me()</computeroutput>, add up
    the counts from each entry.</para>

    <para>The reason for this is that although the debug info
    output by gcc indicates the switch from
    <filename>bar.c</filename> to <filename>foo.h</filename>, it
    doesn't indicate the name of the function in
    <filename>foo.h</filename>, so Valgrind keeps using the old
    one.</para>
  </listitem>

  <listitem>
    <para>Sometimes, the same filename might be represented with
    a relative name and with an absolute name in different parts
    of the debug info, eg:
    <filename>/home/user/proj/proj.h</filename> and
    <filename>../proj.h</filename>.  In this case, if you use
    auto-annotation, the file will be annotated twice with the
    counts split between the two.</para>
  </listitem>

  <listitem>
    <para>Files with more than 65,535 lines cause difficulties
    for the stabs debug info reader.  This is because the line
    number in the <computeroutput>struct nlist</computeroutput>
    defined in <filename>a.out.h</filename> under Linux is only a
    16-bit value.  Valgrind can handle some files with more than
    65,535 lines correctly by making some guesses to identify
    line number overflows.  But some cases are beyond it, in
    which case you'll get a warning message explaining that
    annotations for the file might be incorrect.</para>
  </listitem>

  <listitem>
    <para>If you compile some files with
    <computeroutput>-g</computeroutput> and some without, some
    events that take place in a file without debug info could be
    attributed to the last line of a file with debug info
    (whichever one gets placed before the non-debug-info file in
    the executable).</para>
  </listitem>

</itemizedlist>

<para>This list looks long, but these cases should be fairly
rare.</para>

<formalpara>
  <title>Note:</title>
  <para><computeroutput>stabs</computeroutput> is not an easy
  format to read.  If you come across bizarre annotations that
  look like might be caused by a bug in the stabs reader, please
  let us know.</para>
</formalpara>

</sect2>



<sect2>
<title>Accuracy</title>

<para>Valgrind's cache profiling has a number of
shortcomings:</para>

<itemizedlist>
  <listitem>
    <para>It doesn't account for kernel activity -- the effect of
    system calls on the cache contents is ignored.</para>
  </listitem>

  <listitem>
    <para>It doesn't account for other process activity (although
    this is probably desirable when considering a single
    program).</para>
  </listitem>

  <listitem>
    <para>It doesn't account for virtual-to-physical address
    mappings; hence the entire simulation is not a true
    representation of what's happening in the
    cache.</para>
  </listitem>

  <listitem>
    <para>It doesn't account for cache misses not visible at the
    instruction level, eg. those arising from TLB misses, or
    speculative execution.</para>
  </listitem>

  <listitem>
    <para>Valgrind will schedule
    threads differently from how they would be when running natively.
    This could warp the results for threaded programs.</para>
  </listitem>

  <listitem>
    <para>The x86/amd64 instructions <computeroutput>bts</computeroutput>,
    <computeroutput>btr</computeroutput> and
    <computeroutput>btc</computeroutput> will incorrectly be
    counted as doing a data read if both the arguments are
    registers, eg:</para>
<programlisting><![CDATA[
    btsl %eax, %edx]]></programlisting>

    <para>This should only happen rarely.</para>
  </listitem>

  <listitem>
    <para>x86/amd64 FPU instructions with data sizes of 28 and 108 bytes
    (e.g.  <computeroutput>fsave</computeroutput>) are treated as
    though they only access 16 bytes.  These instructions seem to
    be rare so hopefully this won't affect accuracy much.</para>
  </listitem>

</itemizedlist>

<para>Another thing worth nothing is that results are very
sensitive.  Changing the size of the
<filename>valgrind.so</filename> file, the size of the program
being profiled, or even the length of its name can perturb the
results.  Variations will be small, but don't expect perfectly
repeatable results if your program changes at all.</para>

<para>While these factors mean you shouldn't trust the results to
be super-accurate, hopefully they should be close enough to be
useful.</para>

</sect2>


<sect2>
<title>Todo</title>

<itemizedlist>
  <listitem>
    <para>Program start-up/shut-down calls a lot of functions
    that aren't interesting and just complicate the output.
    Would be nice to exclude these somehow.</para>
  </listitem>
</itemizedlist> 

</sect2>

</sect1>
</chapter>