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Add support for combining GEPs across PHI nodes

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Currently LLVM will generally merge GEPs. This allows backends to use more
complex addressing modes. In some cases this is not happening because there
is PHI inbetween the two GEPs:

  GEP1--\
        |-->PHI1-->GEP3
  GEP2--/

This patch checks to see if GEP1 and GEP2 are similiar enough that they can be
cloned (GEP12) in GEP3's BB, allowing GEP->GEP merging (GEP123):

  GEP1--\                     --\                           --\
        |-->PHI1-->GEP3  ==>    |-->PHI2->GEP12->GEP3 == >    |-->PHI2->GEP123
  GEP2--/                     --/                           --/

This also breaks certain use chains that are preventing GEP->GEP merges that the
the existing instcombine would merge otherwise.

Tests included.

rdar://15547484

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209049 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Louis Gerbarg 2014-05-16 23:47:24 +00:00
parent 70f8b47a65
commit 2d1fa5f718
2 changed files with 115 additions and 0 deletions
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59
lib/Transforms/InstCombine/InstructionCombining.cpp
View File

@ -1220,6 +1220,65 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
if (MadeChange) return &GEP;
}
// Check to see if the inputs to the PHI node are getelementptr instructions.
if (PHINode *PN = dyn_cast<PHINode>(PtrOp)) {
GetElementPtrInst *Op1 = dyn_cast<GetElementPtrInst>(PN->getOperand(0));
if (!Op1)
return nullptr;
signed DI = -1;
for (auto I = PN->op_begin()+1, E = PN->op_end(); I !=E; ++I) {
GetElementPtrInst *Op2 = dyn_cast<GetElementPtrInst>(*I);
if (!Op2 || Op1->getNumOperands() != Op1->getNumOperands())
return nullptr;
for (unsigned J = 0, F = Op1->getNumOperands(); J != F; ++J) {
if (Op1->getOperand(J) != Op2->getOperand(J)) {
if (DI == -1) {
// We have not seen any differences yet in the GEPs feeding the
// PHI yet, so we record this one.
DI = J;
} else {
// The GEP is different by more than one input. While this could be
// extended to support GEPs that vary by more than one variable it
// doesn't make sense since it greatly increases the complexity and
// would result in an R+R+R addressing mode which no backend
// directly supports and would need to be broken into several
// simpler instructions anyway.
return nullptr;
}
}
}
}
GetElementPtrInst *NewGEP = dyn_cast<GetElementPtrInst>(Op1->clone());
if (DI == -1) {
// All the GEPs feeding the PHI are identical. Clone one down into our
// BB so that it can be merged with the current GEP.
GEP.getParent()->getInstList().insert(GEP.getParent()->getFirstNonPHI(),
NewGEP);
} else {
// All the GEPs feeding the PHI differ at a single offset. Clone a GEP
// into the current block so it can be merged, and create a new PHI to
// set that index.
PHINode *NewPN = Builder->CreatePHI(Op1->getOperand(DI)->getType(),
PN->getNumOperands());
for (auto &I : PN->operands())
NewPN->addIncoming(dyn_cast<GEPOperator>(I)->getOperand(DI),
PN->getIncomingBlock(I));
NewGEP->setOperand(DI, NewPN);
GEP.getParent()->getInstList().insert(GEP.getParent()->getFirstNonPHI(),
NewGEP);
NewGEP->setOperand(DI, NewPN);
}
GEP.setOperand(0, NewGEP);
PtrOp = NewGEP;
}
// Combine Indices - If the source pointer to this getelementptr instruction
// is a getelementptr instruction, combine the indices of the two
// getelementptr instructions into a single instruction.

56
test/Transforms/InstCombine/gepphigep.ll Normal file
View File

@ -0,0 +1,56 @@
; RUN: opt -instcombine -S < %s | FileCheck %s
%struct1 = type { %struct2*, i32, i32, i32 }
%struct2 = type { i32, i32 }
define i32 @test1(%struct1* %dm, i1 %tmp4, i64 %tmp9, i64 %tmp19) {
bb:
%tmp = getelementptr inbounds %struct1* %dm, i64 0, i32 0
%tmp1 = load %struct2** %tmp, align 8
br i1 %tmp4, label %bb1, label %bb2
bb1:
%tmp10 = getelementptr inbounds %struct2* %tmp1, i64 %tmp9
%tmp11 = getelementptr inbounds %struct2* %tmp10, i64 0, i32 0
store i32 0, i32* %tmp11, align 4
br label %bb3
bb2:
%tmp20 = getelementptr inbounds %struct2* %tmp1, i64 %tmp19
%tmp21 = getelementptr inbounds %struct2* %tmp20, i64 0, i32 0
store i32 0, i32* %tmp21, align 4
br label %bb3
bb3:
%phi = phi %struct2* [ %tmp10, %bb1 ], [ %tmp20, %bb2 ]
%tmp24 = getelementptr inbounds %struct2* %phi, i64 0, i32 1
%tmp25 = load i32* %tmp24, align 4
ret i32 %tmp25
; CHECK-LABEL: @test1(
; CHECK: getelementptr inbounds %struct2* %tmp1, i64 %tmp9, i32 0
; CHECK: getelementptr inbounds %struct2* %tmp1, i64 %tmp19, i32 0
; CHECK: %[[PHI:[0-9A-Za-z]+]] = phi i64 [ %tmp9, %bb1 ], [ %tmp19, %bb2 ]
; CHECK: getelementptr inbounds %struct2* %tmp1, i64 %[[PHI]], i32 1
}
define i32 @test2(%struct1* %dm, i1 %tmp4, i64 %tmp9, i64 %tmp19) {
bb:
%tmp = getelementptr inbounds %struct1* %dm, i64 0, i32 0
%tmp1 = load %struct2** %tmp, align 8
%tmp10 = getelementptr inbounds %struct2* %tmp1, i64 %tmp9
%tmp11 = getelementptr inbounds %struct2* %tmp10, i64 0, i32 0
store i32 0, i32* %tmp11, align 4
%tmp20 = getelementptr inbounds %struct2* %tmp1, i64 %tmp19
%tmp21 = getelementptr inbounds %struct2* %tmp20, i64 0, i32 0
store i32 0, i32* %tmp21, align 4
%tmp24 = getelementptr inbounds %struct2* %tmp10, i64 0, i32 1
%tmp25 = load i32* %tmp24, align 4
ret i32 %tmp25
; CHECK-LABEL: @test2(
; CHECK: getelementptr inbounds %struct2* %tmp1, i64 %tmp9, i32 0
; CHECK: getelementptr inbounds %struct2* %tmp1, i64 %tmp19, i32 0
; CHECK: getelementptr inbounds %struct2* %tmp1, i64 %tmp9, i32 1
}