[InstSimplify] Correctly handle comparison with zero-size allocs

InstSimplify currently folds alloc1 == alloc2 to false, even if
one of them is a zero-size allocation. A zero-size allocation may
have the same address as another allocation.

This also disables the fold for the case where we're comparing a
zero-size alloc with the middle of another allocation. It's
possible that this case is legal to fold depending on our precise
zero-size allocation semantics, but LangRef currently doesn't
specify this either way, so we shouldn't make assumptions here.

Author: nikic

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -2774,8 +2774,8 @@ static Constant *computePointerICmp(CmpInst::Predicate Pred, Value *LHS,
         return nullptr;
       }(LHS);
       Opts.NullIsUnknownSize = F ? NullPointerIsDefined(F) : true;
-      if (getObjectSize(LHS, LHSSize, DL, TLI, Opts) &&
-          getObjectSize(RHS, RHSSize, DL, TLI, Opts)) {
+      if (getObjectSize(LHS, LHSSize, DL, TLI, Opts) && LHSSize != 0 &&
+          getObjectSize(RHS, RHSSize, DL, TLI, Opts) && RHSSize != 0) {
         APInt Dist = LHSOffset - RHSOffset;
         if (Dist.isNonNegative() ? Dist.ult(LHSSize) : (-Dist).ult(RHSSize))
           return ConstantInt::get(getCompareTy(LHS),

llvm/test/Transforms/InstSimplify/cmp-alloca-offsets.ll

@@ -234,8 +234,9 @@ define i1 @zst_alloca_start() {
 ; CHECK-LABEL: @zst_alloca_start(
 ; CHECK-NEXT:    [[A:%.*]] = alloca i64, align 8
 ; CHECK-NEXT:    [[A2:%.*]] = alloca {}, align 8
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq ptr [[A]], [[A2]]
 ; CHECK-NEXT:    call void @escape(ptr [[A]], ptr [[A2]])
-; CHECK-NEXT:    ret i1 false
+; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %a = alloca i64
   %a2 = alloca {}
@@ -249,8 +250,10 @@ define i1 @zst_alloca_middle() {
 ; CHECK-LABEL: @zst_alloca_middle(
 ; CHECK-NEXT:    [[A:%.*]] = alloca i64, align 8
 ; CHECK-NEXT:    [[A2:%.*]] = alloca {}, align 8
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i8, ptr [[A]], i64 4
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq ptr [[GEP]], [[A2]]
 ; CHECK-NEXT:    call void @escape(ptr [[A]], ptr [[A2]])
-; CHECK-NEXT:    ret i1 false
+; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %a = alloca i64
   %a2 = alloca {}
@@ -282,8 +285,9 @@ define i1 @zst_alloca_end() {
 define i1 @zst_global_start() {
 ; CHECK-LABEL: @zst_global_start(
 ; CHECK-NEXT:    [[A:%.*]] = alloca i64, align 8
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq ptr [[A]], @gz
 ; CHECK-NEXT:    call void @escape(ptr [[A]], ptr @gz)
-; CHECK-NEXT:    ret i1 false
+; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %a = alloca i64
   %gep = getelementptr i8, ptr %a, i64 0
@@ -295,8 +299,10 @@ define i1 @zst_global_start() {
 define i1 @zst_global_middle() {
 ; CHECK-LABEL: @zst_global_middle(
 ; CHECK-NEXT:    [[A:%.*]] = alloca i64, align 8
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i8, ptr [[A]], i64 4
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq ptr [[GEP]], @gz
 ; CHECK-NEXT:    call void @escape(ptr [[A]], ptr @gz)
-; CHECK-NEXT:    ret i1 false
+; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %a = alloca i64
   %gep = getelementptr i8, ptr %a, i64 4