[InferAttrs] Do not mark first argument of str(n)cat as writeonly.

str(n)cat appends a copy of the second argument to the end of the first
argument. To find the end of the first argument, str(n)cat has to read
from it until it finds the terminating 0. So it should not be marked as
writeonly. I think this means the argument should not be marked as
writeonly.

(This is causing a mis-compile with legacy DSE, before it got removed)

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D100601

Author: fhahn

llvm/lib/Transforms/Utils/BuildLibCalls.cpp

@@ -257,11 +257,19 @@ bool llvm::inferLibFuncAttributes(Function &F, const TargetLibraryInfo &TLI) {
     Changed |= setDoesNotCapture(F, 1);
     Changed |= setOnlyReadsMemory(F, 0);
     return Changed;
-  case LibFunc_strcpy:
-  case LibFunc_strncpy:
   case LibFunc_strcat:
   case LibFunc_strncat:
+    Changed |= setOnlyAccessesArgMemory(F);
+    Changed |= setDoesNotThrow(F);
     Changed |= setWillReturn(F);
+    Changed |= setReturnedArg(F, 0);
+    Changed |= setDoesNotCapture(F, 1);
+    Changed |= setOnlyReadsMemory(F, 1);
+    Changed |= setDoesNotAlias(F, 0);
+    Changed |= setDoesNotAlias(F, 1);
+    return Changed;
+  case LibFunc_strcpy:
+  case LibFunc_strncpy:
     Changed |= setReturnedArg(F, 0);
     LLVM_FALLTHROUGH;
   case LibFunc_stpcpy:

llvm/test/Transforms/InferFunctionAttrs/annotate.ll

@@ -839,7 +839,7 @@ declare i8* @stpncpy(i8*, i8*, i64)
 ; CHECK: declare i32 @strcasecmp(i8* nocapture, i8* nocapture) [[NOFREE_NOUNWIND_READONLY_WILLRETURN:#[0-9]+]]
 declare i32 @strcasecmp(i8*, i8*)
 
-; CHECK: declare i8* @strcat(i8* noalias returned writeonly, i8* noalias nocapture readonly) [[ARGMEMONLY_NOFREE_NOUNWIND_WILLRETURN]]
+; CHECK: declare i8* @strcat(i8* noalias returned, i8* noalias nocapture readonly) [[ARGMEMONLY_NOFREE_NOUNWIND_WILLRETURN]]
 declare i8* @strcat(i8*, i8*)
 
 ; CHECK: declare i8* @strchr(i8*, i32) [[ARGMEMONLY_NOFREE_NOUNWIND_READONLY]]
@@ -866,7 +866,7 @@ declare i64 @strlen(i8*)
 ; CHECK: declare i32 @strncasecmp(i8* nocapture, i8* nocapture, i64) [[NOFREE_NOUNWIND_READONLY_WILLRETURN]]
 declare i32 @strncasecmp(i8*, i8*, i64)
 
-; CHECK: declare i8* @strncat(i8* noalias returned writeonly, i8* noalias nocapture readonly, i64) [[ARGMEMONLY_NOFREE_NOUNWIND_WILLRETURN]]
+; CHECK: declare i8* @strncat(i8* noalias returned, i8* noalias nocapture readonly, i64) [[ARGMEMONLY_NOFREE_NOUNWIND_WILLRETURN]]
 declare i8* @strncat(i8*, i8*, i64)
 
 ; CHECK: declare i32 @strncmp(i8* nocapture, i8* nocapture, i64) [[ARGMEMONLY_NOFREE_NOUNWIND_READONLY]]