[PATCH] D19390: folding compares for distinct allocations

[Anna Thomas via llvm-commits]

anna removed rL LLVM as the repository for this revision.
anna updated this revision to Diff 54644.
anna added a comment.

Added tests to catch cases in future when ordering of optimization changes (or changes to `LikeAllocFn` with `LookThroughBitCast` is set to true) which causes `LikeAllocFn`  to return true. 
Also, improved the check to see if 2 allocations are actually of the same pointer, by stripping through bitcasts and zero GEPs.


http://reviews.llvm.org/D19390

Files:
  lib/Transforms/InstCombine/InstructionCombining.cpp
  test/Transforms/InstCombine/compare-unescaped.ll

Index: test/Transforms/InstCombine/compare-unescaped.ll
===================================================================
--- test/Transforms/InstCombine/compare-unescaped.ll
+++ test/Transforms/InstCombine/compare-unescaped.ll
@@ -40,3 +40,53 @@
   ret i32 %rt 
 ; CHECK: ret i32 %rt
 }
+
+define i1 @compare_distinct_mallocs() {
+  %m = call i8* @malloc(i64 4)
+  %n = call i8* @malloc(i64 4)
+  %cmp = icmp eq i8* %m, %n
+  ret i1 %cmp
+  ; CHECK-LABEL: compare_distinct_mallocs
+  ; CHECK: ret i1 false
+}
+
+; the compare is folded to true since the folding compare looks through bitcasts. 
+; call to malloc and the bitcast instructions are elided after that since there are no uses of the malloc 
+define i1 @compare_samepointer_under_bitcast() {
+  %m = call i8* @malloc(i64 4)
+  %bc = bitcast i8* %m to i32*
+  %bcback = bitcast i32* %bc to i8*
+  %cmp = icmp eq i8* %m, %bcback
+  ret i1 %cmp
+; CHECK-LABEL: compare_samepointer_under_bitcast
+; CHECK: ret i1 true 
+}
+
+; the compare is folded to true since the folding compare looks through bitcasts. 
+; call to malloc and the bitcast instructions are elided after that since there are no uses of the malloc 
+define i1 @compare_samepointer_escaped() {
+  %m = call i8* @malloc(i64 4)
+  %bc = bitcast i8* %m to i32*
+  %bcback = bitcast i32* %bc to i8*
+  %cmp = icmp eq i8* %m, %bcback
+  call void @f() [ "deopt"(i8* %m) ]
+  ret i1 %cmp
+; CHECK-LABEL: compare_samepointer_escaped
+; CHECK_NEXT: %m = call i8* @malloc(i64 4)
+; CHECK_NEXT: call void @f() [ "deopt"(i8* %m) ]
+; CHECK: ret i1 true 
+}
+
+; The malloc call for %m cannot be elided since it is used in the call to function f.
+; However, the cmp can be folded to true as %n doesnt escape and %m, %n are distinct allocations
+define i1 @compare_distinct_pointer_escape() {
+  %m = call i8* @malloc(i64 4)
+  %n = call i8* @malloc(i64 4)
+  tail call void @f() [ "deopt"(i8* %m) ]
+  %cmp = icmp ne i8* %m, %n
+  ret i1 %cmp
+; CHECK-LABEL: compare_distinct_pointer_escape
+; CHECK_NEXT: %m = call i8* @malloc(i64 4)
+; CHECK_NEXT: tail call void @f() [ "deopt"(i8* %m) ]
+; CHECK_NEXT: ret i1 true
+}
Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1863,12 +1863,14 @@
   return nullptr;
 }
 
-static bool isNeverEqualToUnescapedAlloc(Value *V) {
+static bool isNeverEqualToUnescapedAlloc(Value *V,
+                                         const TargetLibraryInfo *TLI) {
   if (isa<ConstantPointerNull>(V))
     return true;
   if (auto *LI = dyn_cast<LoadInst>(V))
     return isa<GlobalVariable>(LI->getPointerOperand());
-  return false;
+  // Two distinct allocations will never be equal.
+  return isAllocLikeFn(V, TLI);
 }
 
 static bool
@@ -1900,7 +1902,12 @@
         if (!ICI->isEquality())
           return false;
         unsigned OtherIndex = (ICI->getOperand(0) == PI) ? 1 : 0;
-        if (!isNeverEqualToUnescapedAlloc(ICI->getOperand(OtherIndex)))
+        // We avoid the case of allocation being compared to itself.
+        // Strip off pointer and zero GEPs
+        if (PI ==
+                ICI->getOperand(OtherIndex)
+                    ->stripPointerCastsNoFollowAliases() ||
+            !isNeverEqualToUnescapedAlloc(ICI->getOperand(OtherIndex), TLI))
           return false;
         Users.emplace_back(I);
         continue;


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