[PATCH] D18939: [AliasSetTracker] Correctly handle changing size of an entry

[Michael Kuperstein via llvm-commits]

mkuper created this revision.
mkuper added reviewers: spatel, hfinkel.
mkuper added a subscriber: llvm-commits.

If the size of an AST entry changes, we also need to make sure we perform necessary alias set merges, as the new size may overlap pointers in other sets.
We happen to run into this with memset, because memset allows an entry for a i8* pointer to have a decidedly non-i8 size.

There's some ugliness here because even if getEntryFor(Pointer).hasAliasSet() is true, findAliasSetForPointer(Pointer, ...) will not necessarily find an alias set - as a pointer may not alias anything, including itself (e.g. undef). I'm not sure whether that is a bug or a feature...

This fixes PR27262.

http://reviews.llvm.org/D18939

Files:
  include/llvm/Analysis/AliasSetTracker.h
  lib/Analysis/AliasSetTracker.cpp
  test/Transforms/LICM/pr27262.ll

Index: test/Transforms/LICM/pr27262.ll
===================================================================
--- test/Transforms/LICM/pr27262.ll
+++ test/Transforms/LICM/pr27262.ll
@@ -0,0 +1,33 @@
+; RUN: opt -S -basicaa -licm < %s | FileCheck %s
+
+target datalayout = "e-m:x-p:32:32-i64:64-f80:32-n8:16:32-a:0:32-S32"
+target triple = "i686-pc-windows-msvc18.0.0"
+
+; Make sure the store to v is not sunk past the memset
+; CHECK-LABEL: @main
+; CHECK: for.body:
+; CHECK-NEXT: store i8 1, i8* %p
+; CHECK-NEXT: store i8 2, i8* %p1
+; CHECK-NEXT: call void @llvm.memset
+; CHECK: end:
+; CHECK-NEXT: ret i32 0
+
+define i32 @main(i1 %k, i8* %p) {
+entry:
+  %p1 = getelementptr i8, i8* %p, i32 1
+  br label %for.body
+ 
+for.body:
+  store i8 1, i8* %p, align 1
+  store i8 2, i8* %p1, align 1
+  call void @llvm.memset.p0i8.i32(i8* %p, i8 255, i32 4, i32 1, i1 false)  
+  br label %for.latch
+  
+for.latch:
+  br i1 %k, label %for.body, label %end
+
+end:
+  ret i32 0
+}
+
+declare void @llvm.memset.p0i8.i32(i8* nocapture, i8, i32, i32, i1)
Index: include/llvm/Analysis/AliasSetTracker.h
===================================================================
--- include/llvm/Analysis/AliasSetTracker.h
+++ include/llvm/Analysis/AliasSetTracker.h
@@ -59,15 +59,21 @@
       return &NextInList;
     }
 
-    void updateSizeAndAAInfo(uint64_t NewSize, const AAMDNodes &NewAAInfo) {
-      if (NewSize > Size) Size = NewSize;
+    bool updateSizeAndAAInfo(uint64_t NewSize, const AAMDNodes &NewAAInfo) {
+      bool SizeChanged = false;
+      if (NewSize > Size) {
+        Size = NewSize;
+        SizeChanged = true;
+      }
 
       if (AAInfo == DenseMapInfo<AAMDNodes>::getEmptyKey())
         // We don't have a AAInfo yet. Set it to NewAAInfo.
         AAInfo = NewAAInfo;
       else if (AAInfo != NewAAInfo)
         // NewAAInfo conflicts with AAInfo.
         AAInfo = DenseMapInfo<AAMDNodes>::getTombstoneKey();
+
+      return SizeChanged;
     }
 
     uint64_t getSize() const { return Size; }
Index: lib/Analysis/AliasSetTracker.cpp
===================================================================
--- lib/Analysis/AliasSetTracker.cpp
+++ lib/Analysis/AliasSetTracker.cpp
@@ -274,7 +274,14 @@
 
   // Check to see if the pointer is already known.
   if (Entry.hasAliasSet()) {
-    Entry.updateSizeAndAAInfo(Size, AAInfo);
+    // If the size changed, we may need to merge several alias sets.
+    // Use findAliasSetForPointer for its merging side effect.
+    // Note that we can *not* return the result of findAliasSetForPointer
+    // due to a quirk of alias analysis behavior. Since alias(undef, undef)
+    // is NoAlias, findAliasSetForPointer(undef, ...) will not find the
+    // the right set for undef, even if it exists.
+    if (Entry.updateSizeAndAAInfo(Size, AAInfo))
+      findAliasSetForPointer(Pointer, Size, AAInfo);
     // Return the set!
     return *Entry.getAliasSet(*this)->getForwardedTarget(*this);
   }


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