[llvm] 7eec832 - [DSE] Improve handling of `strncpy` in Dead Store Elimination

[Philip Reames via llvm-commits]

JFYI, it looks like the routine in DSE could be sunk into/merged with 
the MemoryLocation apis.  Consolidating code would let other consumers 
benefit as well.

Philip

On 11/19/21 9:53 AM, Florian Hahn via llvm-commits wrote:
> Author: Fabian Wolff
> Date: 2021-11-19T17:46:29Z
> New Revision: 7eec832def5717b1bddb72c3b99c3df4f7a2f6da
>
> URL: https://github.com/llvm/llvm-project/commit/7eec832def5717b1bddb72c3b99c3df4f7a2f6da
> DIFF: https://github.com/llvm/llvm-project/commit/7eec832def5717b1bddb72c3b99c3df4f7a2f6da.diff
>
> LOG: [DSE] Improve handling of `strncpy` in Dead Store Elimination
>
> Fixes PR#52062 and one of the remaining cases of PR#47644.
>
> Reviewed By: fhahn
>
> Differential Revision: https://reviews.llvm.org/D114035
>
> Added:
>      
>
> Modified:
>      llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
>      llvm/test/Transforms/DeadStoreElimination/libcalls.ll
>
> Removed:
>      
>
>
> ################################################################################
> diff  --git a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
> index f2268fe762450..a8ec8bb979707 100644
> --- a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
> +++ b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
> @@ -1059,8 +1059,13 @@ struct DSEState {
>         LibFunc LF;
>         if (TLI.getLibFunc(*CB, LF) && TLI.has(LF)) {
>           switch (LF) {
> -        case LibFunc_strcpy:
>           case LibFunc_strncpy:
> +          if (const auto *Len = dyn_cast<ConstantInt>(CB->getArgOperand(2)))
> +            return MemoryLocation(CB->getArgOperand(0),
> +                                  LocationSize::precise(Len->getZExtValue()),
> +                                  CB->getAAMetadata());
> +          LLVM_FALLTHROUGH;
> +        case LibFunc_strcpy:
>           case LibFunc_strcat:
>           case LibFunc_strncat:
>             return {MemoryLocation::getAfter(CB->getArgOperand(0))};
>
> diff  --git a/llvm/test/Transforms/DeadStoreElimination/libcalls.ll b/llvm/test/Transforms/DeadStoreElimination/libcalls.ll
> index 0639f95b5eb2a..afe064daf5bd5 100644
> --- a/llvm/test/Transforms/DeadStoreElimination/libcalls.ll
> +++ b/llvm/test/Transforms/DeadStoreElimination/libcalls.ll
> @@ -435,8 +435,7 @@ declare void @llvm.memset.p0i8.i64(i8* nocapture, i8, i64, i1) nounwind
>   ; strncpy -> memset, full overwrite
>   define void @dse_strncpy_test1(i8* noalias %out, i8* noalias %in) {
>   ; CHECK-LABEL: @dse_strncpy_test1(
> -; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @strncpy(i8* [[OUT:%.*]], i8* [[IN:%.*]], i64 100)
> -; CHECK-NEXT:    tail call void @llvm.memset.p0i8.i64(i8* [[OUT]], i8 42, i64 100, i1 false)
> +; CHECK-NEXT:    tail call void @llvm.memset.p0i8.i64(i8* [[OUT:%.*]], i8 42, i64 100, i1 false)
>   ; CHECK-NEXT:    ret void
>   ;
>     %call = tail call i8* @strncpy(i8* %out, i8* %in, i64 100)
> @@ -472,8 +471,7 @@ define void @dse_strncpy_test3(i8* noalias %out1, i8* noalias %out2, i8* noalias
>   ; memset -> strncpy, full overwrite
>   define void @dse_strncpy_test4(i8* noalias %out, i8* noalias %in) {
>   ; CHECK-LABEL: @dse_strncpy_test4(
> -; CHECK-NEXT:    tail call void @llvm.memset.p0i8.i64(i8* [[OUT:%.*]], i8 42, i64 100, i1 false)
> -; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @strncpy(i8* [[OUT]], i8* [[IN:%.*]], i64 100)
> +; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @strncpy(i8* [[OUT:%.*]], i8* [[IN:%.*]], i64 100)
>   ; CHECK-NEXT:    ret void
>   ;
>     tail call void @llvm.memset.p0i8.i64(i8* %out, i8 42, i64 100, i1 false)
> @@ -484,7 +482,8 @@ define void @dse_strncpy_test4(i8* noalias %out, i8* noalias %in) {
>   ; memset -> strncpy, partial overwrite
>   define void @dse_strncpy_test5(i8* noalias %out, i8* noalias %in) {
>   ; CHECK-LABEL: @dse_strncpy_test5(
> -; CHECK-NEXT:    tail call void @llvm.memset.p0i8.i64(i8* [[OUT:%.*]], i8 42, i64 100, i1 false)
> +; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds i8, i8* [[OUT:%.*]], i64 99
> +; CHECK-NEXT:    tail call void @llvm.memset.p0i8.i64(i8* align 1 [[TMP1]], i8 42, i64 1, i1 false)
>   ; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @strncpy(i8* [[OUT]], i8* [[IN:%.*]], i64 99)
>   ; CHECK-NEXT:    ret void
>   ;
>
>
>          
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