[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

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))};

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
 ;