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Teach fast-isel to avoid loading a value from memory when it's already

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available in a register. This is pretty primitive, but it reduces the
number of instructions in common testcases by 4%.

llvm-svn: 107380
This commit is contained in:
Dan Gohman 2010-07-01 03:49:38 +00:00
parent 7219aedff5
commit 8022d8e885
3 changed files with 49 additions and 0 deletions
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2
include/llvm/CodeGen/FastISel.h
View File

@ -307,6 +307,8 @@ protected:
}
private:
bool SelectLoad(const User *I);
bool SelectBinaryOp(const User *I, unsigned ISDOpcode);
bool SelectFNeg(const User *I);

24
lib/CodeGen/SelectionDAG/FastISel.cpp
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@ -48,6 +48,7 @@
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Analysis/DebugInfo.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetLowering.h"
@ -715,9 +716,32 @@ FastISel::SelectFNeg(const User *I) {
return true;
}
bool
FastISel::SelectLoad(const User *I) {
LoadInst *LI = const_cast<LoadInst *>(cast<LoadInst>(I));
// For a load from an alloca, make a limited effort to find the value
// already available in a register, avoiding redundant loads.
if (!LI->isVolatile() && isa<AllocaInst>(LI->getPointerOperand())) {
BasicBlock::iterator ScanFrom = LI;
if (const Value *V = FindAvailableLoadedValue(LI->getPointerOperand(),
LI->getParent(), ScanFrom)) {
unsigned ResultReg = getRegForValue(V);
if (ResultReg != 0) {
UpdateValueMap(I, ResultReg);
return true;
}
}
}
return false;
}
bool
FastISel::SelectOperator(const User *I, unsigned Opcode) {
switch (Opcode) {
case Instruction::Load:
return SelectLoad(I);
case Instruction::Add:
return SelectBinaryOp(I, ISD::ADD);
case Instruction::FAdd:

23
test/CodeGen/X86/fast-isel-loads.ll Normal file
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@ -0,0 +1,23 @@
; RUN: llc -march=x86-64 -O0 -asm-verbose=false < %s | FileCheck %s
; Fast-isel shouldn't reload the argument values from the stack.
; CHECK: foo:
; CHECK-NEXT: movq %rdi, -8(%rsp)
; CHECK-NEXT: movq %rsi, -16(%rsp)
; CHECK-NEXT: movsd 128(%rsi,%rdi,8), %xmm0
; CHECK-NEXT: ret
define double @foo(i64 %x, double* %p) nounwind {
entry:
%x.addr = alloca i64, align 8 ; <i64*> [#uses=2]
%p.addr = alloca double*, align 8 ; <double**> [#uses=2]
store i64 %x, i64* %x.addr
store double* %p, double** %p.addr
%tmp = load i64* %x.addr ; <i64> [#uses=1]
%tmp1 = load double** %p.addr ; <double*> [#uses=1]
%add = add nsw i64 %tmp, 16 ; <i64> [#uses=1]
%arrayidx = getelementptr inbounds double* %tmp1, i64 %add ; <double*> [#uses=1]
%tmp2 = load double* %arrayidx ; <double> [#uses=1]
ret double %tmp2
}