Unverified Commit 15383dcd authored by nataliakokoromyti's avatar nataliakokoromyti Committed by GitHub
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[EarlyCSE] Support memset loads (#194268) 

This PR addresses the zero-`memset` case in EarlyCSE as discussed in
#194080. If we do a `memset` of zero and then load back from the same
base pointer, we can fold that load to `null`.
parent a699b25b

llvm/lib/Transforms/Scalar/EarlyCSE.cpp

+26 −0
Original line number Diff line number Diff line
@@ -821,6 +821,12 @@ private: 
            Info.IsVolatile = false;

            break;

          }

        } else if (auto *MI = dyn_cast<MemSetInst>(Inst)) {

          Info.PtrVal = MI->getDest();

          Info.MatchingId = 0;

          Info.ReadMem = false;

          Info.WriteMem = true;

          Info.IsVolatile = MI->isVolatile();

        }

      }

    }
@@ -1226,6 +1232,26 @@ Value *EarlyCSE::getMatchingValue(LoadValue &InVal, ParseMemoryInst &MemInst, 
                                  unsigned CurrentGeneration) {

  if (InVal.DefInst == nullptr)

    return nullptr;

  if (auto *MSI = dyn_cast<MemSetInst>(InVal.DefInst)) {

    if (!MemInst.isLoad() || MemInst.isVolatile() || !MemInst.isUnordered())

      return nullptr;

    if (MSI->isVolatile())

      return nullptr;

    auto *Val = dyn_cast<ConstantInt>(MSI->getValue());

    if (!Val || !Val->isZero())

      return nullptr;

    auto Len = MSI->getLengthInBytes();

    if (!Len)

      return nullptr;

    TypeSize LoadSize = SQ.DL.getTypeStoreSize(MemInst.getValueType());

    if (LoadSize.isScalable() || Len->ult(LoadSize.getFixedValue()))

      return nullptr;

    if (!isOperatingOnInvariantMemAt(MemInst.get(), InVal.Generation) &&

        !isSameMemGeneration(InVal.Generation, CurrentGeneration, InVal.DefInst,

                             MemInst.get()))

      return nullptr;

    return Constant::getNullValue(MemInst.getValueType());

  }

  if (InVal.MatchingId != MemInst.getMatchingId())

    return nullptr;

  // We don't yet handle removing loads with ordering of any kind.

llvm/test/Transforms/EarlyCSE/memset-load.ll

0 → 100644
+92 −0
Original line number Diff line number Diff line 
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6

; RUN: opt -S -passes='early-cse<memssa>' < %s | FileCheck %s



target datalayout = "pe1:64:64:64:32"



define ptr @load_from_zero_memset(ptr %p) {

; CHECK-LABEL: define ptr @load_from_zero_memset(

; CHECK-SAME: ptr [[P:%.*]]) {

; CHECK-NEXT:  [[ENTRY:.*:]]

; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr [[P]], i8 0, i64 8, i1 false)

; CHECK-NEXT:    ret ptr null

;

entry:

  call void @llvm.memset.p0.i64(ptr %p, i8 0, i64 8, i1 false)

  %v = load ptr, ptr %p, align 8

  ret ptr %v

}



define ptr @load_from_nonzero_memset(ptr %p) {

; CHECK-LABEL: define ptr @load_from_nonzero_memset(

; CHECK-SAME: ptr [[P:%.*]]) {

; CHECK-NEXT:  [[ENTRY:.*:]]

; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr [[P]], i8 1, i64 8, i1 false)

; CHECK-NEXT:    [[V:%.*]] = load ptr, ptr [[P]], align 8

; CHECK-NEXT:    ret ptr [[V]]

;

entry:

  call void @llvm.memset.p0.i64(ptr %p, i8 1, i64 8, i1 false)

  %v = load ptr, ptr %p, align 8

  ret ptr %v

}



define ptr @load_from_zero_memset_with_clobber(ptr %p) {

; CHECK-LABEL: define ptr @load_from_zero_memset_with_clobber(

; CHECK-SAME: ptr [[P:%.*]]) {

; CHECK-NEXT:  [[ENTRY:.*:]]

; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr [[P]], i8 0, i64 8, i1 false)

; CHECK-NEXT:    call void @clobber(ptr [[P]])

; CHECK-NEXT:    [[V:%.*]] = load ptr, ptr [[P]], align 8

; CHECK-NEXT:    ret ptr [[V]]

;

entry:

  call void @llvm.memset.p0.i64(ptr %p, i8 0, i64 8, i1 false)

  call void @clobber(ptr %p)

  %v = load ptr, ptr %p, align 8

  ret ptr %v

}



define ptr @load_from_volatile_zero_memset(ptr %p) {

; CHECK-LABEL: define ptr @load_from_volatile_zero_memset(

; CHECK-SAME: ptr [[P:%.*]]) {

; CHECK-NEXT:  [[ENTRY:.*:]]

; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr [[P]], i8 0, i64 8, i1 true)

; CHECK-NEXT:    [[V:%.*]] = load ptr, ptr [[P]], align 8

; CHECK-NEXT:    ret ptr [[V]]

;

entry:

  call void @llvm.memset.p0.i64(ptr %p, i8 0, i64 8, i1 true)

  %v = load ptr, ptr %p, align 8

  ret ptr %v

}



define ptr @load_from_zero_memset_unknown_length(ptr %p, i64 %n) {

; CHECK-LABEL: define ptr @load_from_zero_memset_unknown_length(

; CHECK-SAME: ptr [[P:%.*]], i64 [[N:%.*]]) {

; CHECK-NEXT:  [[ENTRY:.*:]]

; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr [[P]], i8 0, i64 [[N]], i1 false)

; CHECK-NEXT:    [[V:%.*]] = load ptr, ptr [[P]], align 8

; CHECK-NEXT:    ret ptr [[V]]

;

entry:

  call void @llvm.memset.p0.i64(ptr %p, i8 0, i64 %n, i1 false)

  %v = load ptr, ptr %p, align 8

  ret ptr %v

}



define ptr addrspace(1) @load_from_zero_memset_external(ptr addrspace(1) %p) {

; CHECK-LABEL: define ptr addrspace(1) @load_from_zero_memset_external(

; CHECK-SAME: ptr addrspace(1) [[P:%.*]]) {

; CHECK-NEXT:  [[ENTRY:.*:]]

; CHECK-NEXT:    call void @llvm.memset.p1.i64(ptr addrspace(1) [[P]], i8 0, i64 8, i1 false)

; CHECK-NEXT:    ret ptr addrspace(1) null

;

entry:

  call void @llvm.memset.p1.i64(ptr addrspace(1) %p, i8 0, i64 8, i1 false)

  %v = load ptr addrspace(1), ptr addrspace(1) %p, align 8

  ret ptr addrspace(1) %v

}



declare void @llvm.memset.p0.i64(ptr nocapture writeonly, i8, i64, i1 immarg)

declare void @llvm.memset.p1.i64(ptr addrspace(1) nocapture writeonly, i8, i64, i1 immarg)

declare void @clobber(ptr)