[llvm] f62de7c - [SLC] Transform strncpy(dst, "text", C) to memcpy(dst, "text\0\0\0", C) for C <= 128 only

Dávid Bolvanský via llvm-commits llvm-commits at lists.llvm.org
Sat Aug 15 03:01:35 PDT 2020


Right, strncpy does zeroing, so we need to preserve it.

> Dňa 15. 8. 2020 o 5:19 užívateľ Craig Topper <craig.topper at gmail.com> napísal:
> 
> 
> Doesn't strncpy promise that any bytes past the end of the source string are zeroed in the destination?
> 
> ~Craig
> 
> 
>> On Fri, Aug 14, 2020 at 7:55 PM Philip Reames via llvm-commits <llvm-commits at lists.llvm.org> wrote:
>> Looking at your example, I'm confused by the need to pad.  In the 
>> examples, both the source string length and C are constant.  If we know 
>> the source doesn't contain an embedded zero, why not just do a memcpy 
>> for min(Src.length, C)?
>> 
>> Philip
>> 
>> On 8/14/20 4:53 PM, Dávid Bolvanský via llvm-commits wrote:
>> > Author: Dávid Bolvanský
>> > Date: 2020-08-15T01:53:32+02:00
>> > New Revision: f62de7c9c71134af060a3a1686e30e69d439e785
>> >
>> > URL: https://github.com/llvm/llvm-project/commit/f62de7c9c71134af060a3a1686e30e69d439e785
>> > DIFF: https://github.com/llvm/llvm-project/commit/f62de7c9c71134af060a3a1686e30e69d439e785.diff
>> >
>> > LOG: [SLC] Transform strncpy(dst, "text", C) to memcpy(dst, "text\0\0\0", C) for C <= 128 only
>> >
>> > Transformation creates big strings for big C values, so bail out for C > 128.
>> >
>> > Reviewed By: efriedma
>> >
>> > Differential Revision: https://reviews.llvm.org/D86004
>> >
>> > Added:
>> >      
>> >
>> > Modified:
>> >      llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
>> >      llvm/test/Transforms/InstCombine/strncpy-3.ll
>> >
>> > Removed:
>> >      
>> >
>> >
>> > ################################################################################
>> > diff  --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
>> > index 1a7654983e9f..db7078b9283f 100644
>> > --- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
>> > +++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
>> > @@ -610,12 +610,16 @@ Value *LibCallSimplifier::optimizeStrNCpy(CallInst *CI, IRBuilderBase &B) {
>> >   
>> >     // strncpy(a, "a", 4) - > memcpy(a, "a\0\0\0", 4)
>> >     if (Len > SrcLen + 1) {
>> > -    StringRef Str;
>> > -    if (!getConstantStringInfo(Src, Str))
>> > +    if (Len <= 128) {
>> > +      StringRef Str;
>> > +      if (!getConstantStringInfo(Src, Str))
>> > +        return nullptr;
>> > +      std::string SrcStr = Str.str();
>> > +      SrcStr.resize(Len, '\0');
>> > +      Src = B.CreateGlobalString(SrcStr, "str");
>> > +    } else {
>> >         return nullptr;
>> > -    std::string SrcStr = Str.str();
>> > -    SrcStr.resize(Len, '\0');
>> > -    Src = B.CreateGlobalString(SrcStr, "str");
>> > +    }
>> >     }
>> >   
>> >     Type *PT = Callee->getFunctionType()->getParamType(0);
>> >
>> > diff  --git a/llvm/test/Transforms/InstCombine/strncpy-3.ll b/llvm/test/Transforms/InstCombine/strncpy-3.ll
>> > index 744f1e4169e0..28d27a4f3396 100644
>> > --- a/llvm/test/Transforms/InstCombine/strncpy-3.ll
>> > +++ b/llvm/test/Transforms/InstCombine/strncpy-3.ll
>> > @@ -38,3 +38,21 @@ define void @fill_with_zeros3(i8* %dst) {
>> >     tail call i8* @strncpy(i8* %dst, i8* getelementptr inbounds ([4 x i8], [4 x i8]* @str3, i64 0, i64 0), i64 4)
>> >     ret void
>> >   }
>> > +
>> > +define void @fill_with_zeros4(i8* %dst) {
>> > +; CHECK-LABEL: @fill_with_zeros4(
>> > +; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* nonnull align 1 dereferenceable(128) [[DST:%.*]], i8* nonnull align 1 dereferenceable(128) getelementptr inbounds ([129 x i8], [129 x i8]* @str.2, i64 0, i64 0), i64 128, i1 false)
>> > +; CHECK-NEXT:    ret void
>> > +;
>> > +  tail call i8* @strncpy(i8* %dst, i8* getelementptr inbounds ([4 x i8], [4 x i8]* @str3, i64 0, i64 0), i64 128)
>> > +  ret void
>> > +}
>> > +
>> > +define void @no_simplify(i8* %dst) {
>> > +; CHECK-LABEL: @no_simplify(
>> > +; CHECK-NEXT:    [[TMP1:%.*]] = tail call i8* @strncpy(i8* nonnull dereferenceable(1) [[DST:%.*]], i8* nonnull dereferenceable(5) getelementptr inbounds ([4 x i8], [4 x i8]* @str3, i64 0, i64 0), i64 129)
>> > +; CHECK-NEXT:    ret void
>> > +;
>> > +  tail call i8* @strncpy(i8* %dst, i8* getelementptr inbounds ([4 x i8], [4 x i8]* @str3, i64 0, i64 0), i64 129)
>> > +  ret void
>> > +}
>> >
>> >
>> >          
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