Revert "[BasicAA] Remove unneeded special case for malloc/calloc"

This reverts commit 9b1e00738c5ddba681e17e5cb7c260d9afc4c3a7.

Nikic reported in commit thread that I had forgotten history here, and that a) we'd tried this before, and b) had to revert due to an unexpected codegen impact.  Current measurements confirm the same issue still exists.

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 63d5842..cfb3468 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -994,6 +994,20 @@ ModRefInfo BasicAAResult::getModRefInfo(const CallBase *Call,
     }
   }
 
+  // If the call is malloc/calloc like, we can assume that it doesn't
+  // modify any IR visible value.  This is only valid because we assume these
+  // routines do not read values visible in the IR.  TODO: Consider special
+  // casing realloc and strdup routines which access only their arguments as
+  // well.  Or alternatively, replace all of this with inaccessiblememonly once
+  // that's implemented fully.
+  if (isMallocOrCallocLikeFn(Call, &TLI)) {
+    // Be conservative if the accessed pointer may alias the allocation -
+    // fallback to the generic handling below.
+    if (getBestAAResults().alias(MemoryLocation::getBeforeOrAfter(Call), Loc,
+                                 AAQI) == AliasResult::NoAlias)
+      return ModRefInfo::NoModRef;
+  }
+
   // Ideally, there should be no need to special case for memcpy/memove
   // intrinsics here since general machinery (based on memory attributes) should
   // already handle it just fine. Unfortunately, it doesn't due to deficiency in