Subzero: Use register availability during lowering to improve the code.
The problem is that given code like this:
a = b + c
d = a + e
...
... (use of a) ...
Lowering may produce code like this, at least on x86:
T1 = b
T1 += c
a = T1
T2 = a
T2 += e
d = T2
...
... (use of a) ...
If "a" has a long live range, it may not get a register, resulting in clumsy code in the middle of the sequence like "a=reg; reg=a". Normally one might expect store forwarding to make the clumsy code fast, but it does presumably add an extra instruction-retirement cycle to the critical path in a pointer-chasing loop, and makes a big difference on some benchmarks.
The simple fix here is, at the end of lowering "a=b+c", keep track of the final "a=T1" assignment. Then, when lowering "d=a+e" and we look up "a", we can substitute "T1". This slightly increases the live range of T1, but it does a great job of avoiding the redundant reload of the register from the stack location.
A more general fix (in the future) might be to do live range splitting and let the register allocator handle it.
BUG= https://code.google.com/p/nativeclient/issues/detail?id=4095
R=kschimpf@google.com
Review URL: https://codereview.chromium.org/1385433002 .
diff --git a/src/IceTargetLowering.h b/src/IceTargetLowering.h
index 0095a39..f35983f 100644
--- a/src/IceTargetLowering.h
+++ b/src/IceTargetLowering.h
@@ -65,6 +65,9 @@
void setNext(InstList::iterator N) { Next = N; }
void rewind();
void setInsertPoint(const InstList::iterator &Position) { Next = Position; }
+ void availabilityReset();
+ void availabilityUpdate();
+ Variable *availabilityGet(Operand *Src) const;
private:
/// Node is the argument to Inst::updateVars().
@@ -85,6 +88,11 @@
InstList::iterator Begin;
/// End is a copy of Insts.end(), used if Next needs to be advanced.
InstList::iterator End;
+ /// LastDest and LastSrc capture the parameters of the last "Dest=Src" simple
+ /// assignment inserted (provided Src is a variable). This is used for simple
+ /// availability analysis.
+ Variable *LastDest = nullptr;
+ Variable *LastSrc = nullptr;
void skipDeleted(InstList::iterator &I) const;
void advanceForward(InstList::iterator &I) const;