docs/AdvancedGetElementPtr.html - platform/external/llvm - Gitiles

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 <div class="doc_title">
   The Revenge Of The Often Misunderstood GEP Instruction
 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section"><a name="intro"><b>Introduction</b></a></div>
 <!-- *********************************************************************** -->
 <div class="doc_text">
   <p>GEP was mysterious and wily at first, but it turned out that the basic
      workings were fairly comprehensible. However the dragon was merely subdued;
      now it's back, and it has more fundamental complexity to confront. This
      document seeks to uncover misunderstandings of the GEP operator that tend
      to persist past initial confusion about the funky "extra 0" thing.  Here we
      show that the GEP instruction is really not quite as simple as it seems,
      even after the initial confusion is overcome.</p>
 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="lead0"><b>How is GEP different from ptrtoint, arithmetic,
                      and inttoptr?</b></a>
 </div>
 <div class="doc_text">
   <p>It's very similar; there are only subtle differences.</p>

   <p>With ptrtoint, you have to pick an integer type. One approach is to pick i64;
      this is safe on everything LLVM supports (LLVM internally assumes pointers
      are never wider than 64 bits in many places), and the optimizer will actually
      narrow the i64 arithmetic down to the actual pointer size on targets which
      don't support 64-bit arithmetic in most cases. However, there are some cases
      where it doesn't do this. With GEP you can avoid this problem.

   <p>Also, GEP carries additional pointer aliasing rules. It's invalid to take a
      GEP from one object, address into a different separately allocated
      object, and dereference it. IR producers (front-ends) must follow this rule,
      and consumers (optimizers, specifically alias analysis) benefit from being
      able to rely on it.</p>

   <p>And, GEP is more concise in common cases.</p>

   <p>However, for the underlying integer computation implied, there
      is no difference.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="lead0"><b>I'm writing a backend for a target which needs custom
                      lowering for GEP. How do I do this?</b></a>
 </div>
 <div class="doc_text">
   <p>You don't. The integer computation implied by a GEP is target-independent.
      Typically what you'll need to do is make your backend pattern-match
      expressions trees involving ADD, MUL, etc., which are what GEP is lowered
      into. This has the advantage of letting your code work correctly in more
      cases.</p>

   <p>GEP does use target-dependent parameters for the size and layout of data
      types, which targets can customize.</p>

   <p>If you require support for addressing units which are not 8 bits, you'll
      need to fix a lot of code in the backend, with GEP lowering being only a
      small piece of the overall picture.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="lead0"><b>Why do struct member indices always use i32?</b></a>
 </div>
 <div class="doc_text">
   <p>The specific type i32 is probably just a historical artifact, however it's
      wide enough for all practical purposes, so there's been no need to change it.
      It doesn't necessarily imply i32 address arithmetic; it's just an identifier
      which identifies a field in a struct. Requiring that all struct indices be
      the same reduces the range of possibilities for cases where two GEPs are
      effectively the same but have distinct operand types.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="lead0"><b>How does VLA addressing work with GEPs?</b></a>
 </div>
 <div class="doc_text">
   <p>GEPs don't natively support VLAs. LLVM's type system is entirely static,
      and GEP address computations are guided by an LLVM type.</p>

   <p>VLA indices can be implemented as linearized indices. For example, an
      expression like X[a][b][c], must be effectively lowered into a form
      like X[a*m+b*n+c], so that it appears to the GEP as a single-dimensional
      array reference.</p>

   <p>This means if you want to write an analysis which understands array
      indices and you want to support VLAs, your code will have to be
      prepared to reverse-engineer the linearization. One way to solve this
      problem is to use the ScalarEvolution library, which always presents
      VLA and non-VLA indexing in the same manner.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="lead0"><b>What happens if an array index is out of bounds?</b></a>
 </div>
 <div class="doc_text">
   <p>There are two senses in which an array index can be out of bounds.</p>

   <p>First, there's the array type which comes from the (static) type of
      the first operand to the GEP. Indices greater than the number of elements
      in the corresponding static array type are valid. There is no problem with
      out of bounds indices in this sense. Indexing into an array only depends
      on the size of the array element, not the number of elements.</p>

   <p>A common example of how this is used is arrays where the size is not known.
      It's common to use array types with zero length to represent these. The
      fact that the static type says there are zero elements is irrelevant; it's
      perfectly valid to compute arbitrary element indices, as the computation
      only depends on the size of the array element, not the number of
      elements. Note that zero-sized arrays are not a special case here.</p>

   <p>This sense is unconnected with <tt>inbounds</tt> keyword. The
      <tt>inbounds</tt> keyword is designed to describe low-level pointer
      arithmetic overflow conditions, rather than high-level array
      indexing rules.

   <p>Analysis passes which wish to understand array indexing should not
      assume that the static array type bounds are respected.</p>

   <p>The second sense of being out of bounds is computing an address that's
      beyond the actual underlying allocated object.</p>

   <p>With the <tt>inbounds</tt> keyword, the result value of the GEP is
      undefined if the address is outside the actual underlying allocated
      object and not the address one-past-the-end.</p>

   <p>Without the <tt>inbounds</tt> keyword, there are no restrictions
      on computing out-of-bounds addresses. Obviously, performing a load or
      a store requires an address of allocated and sufficiently aligned
      memory. But the GEP itself is only concerned with computing addresses.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="lead0"><b>Can array indices be negative?</b></a>
 </div>
 <div class="doc_text">
   <p>Yes. This is basically a special case of array indices being out
      of bounds.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="lead0"><b>Can I compare two values computed with GEPs?</b></a>
 </div>
 <div class="doc_text">
   <p>Yes. If both addresses are within the same allocated object, or
      one-past-the-end, you'll get the comparison result you expect. If either
      is outside of it, integer arithmetic wrapping may occur, so the
      comparison may not be meaningful.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="lead0"><b>Can I do GEP with a different pointer type than the type of
                      the underlying object?</b></a>
 </div>
 <div class="doc_text">
   <p>Yes. There are no restrictions on bitcasting a pointer value to an arbitrary
      pointer type. The types in a GEP serve only to define the parameters for the
      underlying integer computation. They need not correspond with the actual
      type of the underlying object.</p>

   <p>Furthermore, loads and stores don't have to use the same types as the type
      of the underlying object. Types in this context serve only to specify
      memory size and alignment. Beyond that there are merely a hint to the
      optimizer indicating how the value will likely be used.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="lead0"><b>Can I cast an object's address to integer and add it
                      to null?</b></a>
 </div>
 <div class="doc_text">
   <p>You can compute an address that way, but if you use GEP to do the add,
      you can't use that pointer to actually access the object, unless the
      object is managed outside of LLVM.</p>

   <p>The underlying integer computation is sufficiently defined; null has a
      defined value -- zero -- and you can add whatever value you want to it.</p>

   <p>However, it's invalid to access (load from or store to) an LLVM-aware
      object with such a pointer. This includes GlobalVariables, Allocas, and
      objects pointed to by noalias pointers.</p>

   <p>If you really need this functionality, you can do the arithmetic with
      explicit integer instructions, and use inttoptr to convert the result to
      an address. Most of GEP's special aliasing rules do not apply to pointers
      computed from ptrtoint, arithmetic, and inttoptr sequences.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="lead0"><b>Can I compute the distance between two objects, and add
                      that value to one address to compute the other address?</b></a>
 </div>
 <div class="doc_text">
   <p>As with arithmetic on null, You can use GEP to compute an address that
      way, but you can't use that pointer to actually access the object if you
      do, unless the object is managed outside of LLVM.</p>

   <p>Also as above, ptrtoint and inttoptr provide an alternative way to do this
      which do not have this restriction.</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="lead0"><b>Can I do type-based alias analysis on LLVM IR?</b></a>
 </div>
 <div class="doc_text">
   <p>You can't do type-based alias analysis using LLVM's built-in type system,
      because LLVM has no restrictions on mixing types in addressing, loads or
      stores.</p>

   <p>It would be possible to add special annotations to the IR, probably using
      metadata, to describe a different type system (such as the C type system),
      and do type-based aliasing on top of that. This is a much bigger
      undertaking though.</p>

 </div>

 <!-- *********************************************************************** -->

 <div class="doc_subsection">
   <a name="lead0"><b>What's an uglygep?</b></a>
 </div>
 <div class="doc_text">
   <p>Some LLVM optimizers operate on GEPs by internally lowering them into
      more primitive integer expressions, which allows them to be combined
      with other integer expressions and/or split into multiple separate
      integer expressions. If they've made non-trivial changes, translating
      back into LLVM IR can involve reverse-engineering the structure of
      the addressing in order to fit it into the static type of the original
      first operand. It isn't always possibly to fully reconstruct this
      structure; sometimes the underlying addressing doesn't correspond with
      the static type at all. In such cases the optimizer instead will emit
      a GEP with the base pointer casted to a simple address-unit pointer,
      using the name "uglygep". This isn't pretty, but it's just as
      valid, and it's sufficient to preserve the pointer aliasing guarantees
      that GEP provides.</p>

 </div>

 <!-- *********************************************************************** -->

 <div class="doc_subsection">
   <a name="lead0"><b>Can GEP index into vector elements?</b></a>
 </div>
 <div class="doc_text">
   <p>Sort of. This hasn't always been forcefully disallowed, though it's
      not recommended. It leads to awkward special cases in the optimizers.
      In the future, it may be outright disallowed.</p>

   <p>Instead, you should cast your pointer types and use arrays instead of
      vectors for addressing. Arrays have the same in-memory representation
      as vectors, so the addressing is interchangeable.</p>

 </div>

 <!-- *********************************************************************** -->

 <div class="doc_subsection">
   <a name="lead0"><b>Can GEP index into unions?</b></a>
 </div>
 <div class="doc_text">
    <p>Unknown.</p>

 </div>

 <!-- *********************************************************************** -->

 <div class="doc_subsection">
   <a name="lead0"><b>What happens if a GEP computation overflows?</b></a>
 </div>
 <div class="doc_text">
    <p>If the GEP has the <tt>inbounds</tt> keyword, the result value is
       undefined.</p>

    <p>Otherwise, the result value is the result from evaluating the implied
       two's complement integer computation. However, since there's no
       guarantee of where an object will be allocated in the address space,
       such values have limited meaning.</p>

 </div>

 <!-- *********************************************************************** -->

 <div class="doc_subsection">
   <a name="lead0"><b>What effect do address spaces have on GEPs?</b></a>
 </div>
 <div class="doc_text">
    <p>None, except that the address space qualifier on the first operand pointer
       type always matches the address space qualifier on the result type.</p>

 </div>

 <!-- *********************************************************************** -->

 <div class="doc_subsection">
   <a name="lead0"><b>Why is GEP designed this way?</b></a>
 </div>
 <div class="doc_text">
    <p>The design of GEP has the following goals, in rough unofficial
       order of priority:</p>
    <ul>
      <li>Support C, C-like languages, and languages which can be
          conceptually lowered into C (this covers a lot).</li>
      <li>Support optimizations such as those that are common in
          C compilers.</li>
      <li>Provide a consistent method for computing addresses so that
          address computations don't need to be a part of load and
          store instructions in the IR.</li>
      <li>Support non-C-like languages, to the extent that it doesn't
          interfere with other goals.</li>
      <li>Minimize target-specific information in the IR.</li>
    </ul>
 </div>

 <!-- *********************************************************************** -->

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	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
	"http://www.w3.org/TR/html4/strict.dtd">
	<html>
	<head>
	<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
	<title>The Revenge Of The Often Misunderstood GEP Instruction</title>
	<link rel="stylesheet" href="llvm.css" type="text/css">
	<style type="text/css">
	TABLE { text-align: left; border: 1px solid black; border-collapse: collapse; margin: 0 0 0 0; }
	</style>
	</head>
	<body>

	<div class="doc_title">
	The Revenge Of The Often Misunderstood GEP Instruction
	</div>

	<!-- *********************************************************************** -->
	<div class="doc_section"><a name="intro"><b>Introduction</b></a></div>
	<!-- *********************************************************************** -->
	<div class="doc_text">
	<p>GEP was mysterious and wily at first, but it turned out that the basic
	workings were fairly comprehensible. However the dragon was merely subdued;
	now it's back, and it has more fundamental complexity to confront. This
	document seeks to uncover misunderstandings of the GEP operator that tend
	to persist past initial confusion about the funky "extra 0" thing. Here we
	show that the GEP instruction is really not quite as simple as it seems,
	even after the initial confusion is overcome.</p>
	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="lead0"><b>How is GEP different from ptrtoint, arithmetic,
	and inttoptr?</b></a>
	</div>
	<div class="doc_text">
	<p>It's very similar; there are only subtle differences.</p>

	<p>With ptrtoint, you have to pick an integer type. One approach is to pick i64;
	this is safe on everything LLVM supports (LLVM internally assumes pointers
	are never wider than 64 bits in many places), and the optimizer will actually
	narrow the i64 arithmetic down to the actual pointer size on targets which
	don't support 64-bit arithmetic in most cases. However, there are some cases
	where it doesn't do this. With GEP you can avoid this problem.

	<p>Also, GEP carries additional pointer aliasing rules. It's invalid to take a
	GEP from one object, address into a different separately allocated
	object, and dereference it. IR producers (front-ends) must follow this rule,
	and consumers (optimizers, specifically alias analysis) benefit from being
	able to rely on it.</p>

	<p>And, GEP is more concise in common cases.</p>

	<p>However, for the underlying integer computation implied, there
	is no difference.</p>

	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="lead0"><b>I'm writing a backend for a target which needs custom
	lowering for GEP. How do I do this?</b></a>
	</div>
	<div class="doc_text">
	<p>You don't. The integer computation implied by a GEP is target-independent.
	Typically what you'll need to do is make your backend pattern-match
	expressions trees involving ADD, MUL, etc., which are what GEP is lowered
	into. This has the advantage of letting your code work correctly in more
	cases.</p>

	<p>GEP does use target-dependent parameters for the size and layout of data
	types, which targets can customize.</p>

	<p>If you require support for addressing units which are not 8 bits, you'll
	need to fix a lot of code in the backend, with GEP lowering being only a
	small piece of the overall picture.</p>

	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="lead0"><b>Why do struct member indices always use i32?</b></a>
	</div>
	<div class="doc_text">
	<p>The specific type i32 is probably just a historical artifact, however it's
	wide enough for all practical purposes, so there's been no need to change it.
	It doesn't necessarily imply i32 address arithmetic; it's just an identifier
	which identifies a field in a struct. Requiring that all struct indices be
	the same reduces the range of possibilities for cases where two GEPs are
	effectively the same but have distinct operand types.</p>

	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="lead0"><b>How does VLA addressing work with GEPs?</b></a>
	</div>
	<div class="doc_text">
	<p>GEPs don't natively support VLAs. LLVM's type system is entirely static,
	and GEP address computations are guided by an LLVM type.</p>

	<p>VLA indices can be implemented as linearized indices. For example, an
	expression like X[a][b][c], must be effectively lowered into a form
	like X[am+bn+c], so that it appears to the GEP as a single-dimensional
	array reference.</p>

	<p>This means if you want to write an analysis which understands array
	indices and you want to support VLAs, your code will have to be
	prepared to reverse-engineer the linearization. One way to solve this
	problem is to use the ScalarEvolution library, which always presents
	VLA and non-VLA indexing in the same manner.</p>

	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="lead0"><b>What happens if an array index is out of bounds?</b></a>
	</div>
	<div class="doc_text">
	<p>There are two senses in which an array index can be out of bounds.</p>

	<p>First, there's the array type which comes from the (static) type of
	the first operand to the GEP. Indices greater than the number of elements
	in the corresponding static array type are valid. There is no problem with
	out of bounds indices in this sense. Indexing into an array only depends
	on the size of the array element, not the number of elements.</p>

	<p>A common example of how this is used is arrays where the size is not known.
	It's common to use array types with zero length to represent these. The
	fact that the static type says there are zero elements is irrelevant; it's
	perfectly valid to compute arbitrary element indices, as the computation
	only depends on the size of the array element, not the number of
	elements. Note that zero-sized arrays are not a special case here.</p>

	<p>This sense is unconnected with <tt>inbounds</tt> keyword. The
	<tt>inbounds</tt> keyword is designed to describe low-level pointer
	arithmetic overflow conditions, rather than high-level array
	indexing rules.

	<p>Analysis passes which wish to understand array indexing should not
	assume that the static array type bounds are respected.</p>

	<p>The second sense of being out of bounds is computing an address that's
	beyond the actual underlying allocated object.</p>

	<p>With the <tt>inbounds</tt> keyword, the result value of the GEP is
	undefined if the address is outside the actual underlying allocated
	object and not the address one-past-the-end.</p>

	<p>Without the <tt>inbounds</tt> keyword, there are no restrictions
	on computing out-of-bounds addresses. Obviously, performing a load or
	a store requires an address of allocated and sufficiently aligned
	memory. But the GEP itself is only concerned with computing addresses.</p>

	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="lead0"><b>Can array indices be negative?</b></a>
	</div>
	<div class="doc_text">
	<p>Yes. This is basically a special case of array indices being out
	of bounds.</p>

	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="lead0"><b>Can I compare two values computed with GEPs?</b></a>
	</div>
	<div class="doc_text">
	<p>Yes. If both addresses are within the same allocated object, or
	one-past-the-end, you'll get the comparison result you expect. If either
	is outside of it, integer arithmetic wrapping may occur, so the
	comparison may not be meaningful.</p>

	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="lead0"><b>Can I do GEP with a different pointer type than the type of
	the underlying object?</b></a>
	</div>
	<div class="doc_text">
	<p>Yes. There are no restrictions on bitcasting a pointer value to an arbitrary
	pointer type. The types in a GEP serve only to define the parameters for the
	underlying integer computation. They need not correspond with the actual
	type of the underlying object.</p>

	<p>Furthermore, loads and stores don't have to use the same types as the type
	of the underlying object. Types in this context serve only to specify
	memory size and alignment. Beyond that there are merely a hint to the
	optimizer indicating how the value will likely be used.</p>

	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="lead0"><b>Can I cast an object's address to integer and add it
	to null?</b></a>
	</div>
	<div class="doc_text">
	<p>You can compute an address that way, but if you use GEP to do the add,
	you can't use that pointer to actually access the object, unless the
	object is managed outside of LLVM.</p>

	<p>The underlying integer computation is sufficiently defined; null has a
	defined value -- zero -- and you can add whatever value you want to it.</p>

	<p>However, it's invalid to access (load from or store to) an LLVM-aware
	object with such a pointer. This includes GlobalVariables, Allocas, and
	objects pointed to by noalias pointers.</p>

	<p>If you really need this functionality, you can do the arithmetic with
	explicit integer instructions, and use inttoptr to convert the result to
	an address. Most of GEP's special aliasing rules do not apply to pointers
	computed from ptrtoint, arithmetic, and inttoptr sequences.</p>

	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="lead0"><b>Can I compute the distance between two objects, and add
	that value to one address to compute the other address?</b></a>
	</div>
	<div class="doc_text">
	<p>As with arithmetic on null, You can use GEP to compute an address that
	way, but you can't use that pointer to actually access the object if you
	do, unless the object is managed outside of LLVM.</p>

	<p>Also as above, ptrtoint and inttoptr provide an alternative way to do this
	which do not have this restriction.</p>

	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="lead0"><b>Can I do type-based alias analysis on LLVM IR?</b></a>
	</div>
	<div class="doc_text">
	<p>You can't do type-based alias analysis using LLVM's built-in type system,
	because LLVM has no restrictions on mixing types in addressing, loads or
	stores.</p>

	<p>It would be possible to add special annotations to the IR, probably using
	metadata, to describe a different type system (such as the C type system),
	and do type-based aliasing on top of that. This is a much bigger
	undertaking though.</p>

	</div>

	<!-- *********************************************************************** -->

	<div class="doc_subsection">
	<a name="lead0"><b>What's an uglygep?</b></a>
	</div>
	<div class="doc_text">
	<p>Some LLVM optimizers operate on GEPs by internally lowering them into
	more primitive integer expressions, which allows them to be combined
	with other integer expressions and/or split into multiple separate
	integer expressions. If they've made non-trivial changes, translating
	back into LLVM IR can involve reverse-engineering the structure of
	the addressing in order to fit it into the static type of the original
	first operand. It isn't always possibly to fully reconstruct this
	structure; sometimes the underlying addressing doesn't correspond with
	the static type at all. In such cases the optimizer instead will emit
	a GEP with the base pointer casted to a simple address-unit pointer,
	using the name "uglygep". This isn't pretty, but it's just as
	valid, and it's sufficient to preserve the pointer aliasing guarantees
	that GEP provides.</p>

	</div>

	<!-- *********************************************************************** -->

	<div class="doc_subsection">
	<a name="lead0"><b>Can GEP index into vector elements?</b></a>
	</div>
	<div class="doc_text">
	<p>Sort of. This hasn't always been forcefully disallowed, though it's
	not recommended. It leads to awkward special cases in the optimizers.
	In the future, it may be outright disallowed.</p>

	<p>Instead, you should cast your pointer types and use arrays instead of
	vectors for addressing. Arrays have the same in-memory representation
	as vectors, so the addressing is interchangeable.</p>

	</div>

	<!-- *********************************************************************** -->

	<div class="doc_subsection">
	<a name="lead0"><b>Can GEP index into unions?</b></a>
	</div>
	<div class="doc_text">
	<p>Unknown.</p>

	</div>

	<!-- *********************************************************************** -->

	<div class="doc_subsection">
	<a name="lead0"><b>What happens if a GEP computation overflows?</b></a>
	</div>
	<div class="doc_text">
	<p>If the GEP has the <tt>inbounds</tt> keyword, the result value is
	undefined.</p>

	<p>Otherwise, the result value is the result from evaluating the implied
	two's complement integer computation. However, since there's no
	guarantee of where an object will be allocated in the address space,
	such values have limited meaning.</p>

	</div>

	<!-- *********************************************************************** -->

	<div class="doc_subsection">
	<a name="lead0"><b>What effect do address spaces have on GEPs?</b></a>
	</div>
	<div class="doc_text">
	<p>None, except that the address space qualifier on the first operand pointer
	type always matches the address space qualifier on the result type.</p>

	</div>

	<!-- *********************************************************************** -->

	<div class="doc_subsection">
	<a name="lead0"><b>Why is GEP designed this way?</b></a>
	</div>
	<div class="doc_text">
	<p>The design of GEP has the following goals, in rough unofficial
	order of priority:</p>
	<ul>
	<li>Support C, C-like languages, and languages which can be
	conceptually lowered into C (this covers a lot).</li>
	<li>Support optimizations such as those that are common in
	C compilers.</li>
	<li>Provide a consistent method for computing addresses so that
	address computations don't need to be a part of load and
	store instructions in the IR.</li>
	<li>Support non-C-like languages, to the extent that it doesn't
	interfere with other goals.</li>
	<li>Minimize target-specific information in the IR.</li>
	</ul>
	</div>

	<!-- *********************************************************************** -->

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