[llvm] r209049 - Add support for combining GEPs across PHI nodes

NAKAMURA Takumi geek4civic at gmail.com
Sat May 17 07:55:52 PDT 2014


Reverted in r209067.

It seems this transform would be incompatible to subsequent
optimizations. For example (clang/lib/AST/NSAPI.cpp),

   getelementptr inbounds %"class.clang::NSAPI"* %this, i64 0, i32 %0, i64 0

It is incompatible to gep_type_iterator in
visitGetElementPtrInst:1309, due to non-constant %0.

You can reproduce the failure to build the tree with stage1 clang.

2014-05-17 8:47 GMT+09:00 Louis Gerbarg <lgg at apple.com>:
> Author: louis
> Date: Fri May 16 18:47:24 2014
> New Revision: 209049
>
> URL: http://llvm.org/viewvc/llvm-project?rev=209049&view=rev
> Log:
> Add support for combining GEPs across PHI nodes
>
> 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
>
> Added:
>     llvm/trunk/test/Transforms/InstCombine/gepphigep.ll
> Modified:
>     llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp
>
> Modified: llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp?rev=209049&r1=209048&r2=209049&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp (original)
> +++ llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp Fri May 16 18:47:24 2014
> @@ -1220,6 +1220,65 @@ Instruction *InstCombiner::visitGetEleme
>      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.
>
> Added: llvm/trunk/test/Transforms/InstCombine/gepphigep.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/gepphigep.ll?rev=209049&view=auto
> ==============================================================================
> --- llvm/trunk/test/Transforms/InstCombine/gepphigep.ll (added)
> +++ llvm/trunk/test/Transforms/InstCombine/gepphigep.ll Fri May 16 18:47:24 2014
> @@ -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
> +}
> +
>
>
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