[InstCombine] Fix negative GEP offset evaluation for 32-bit pointers

This fixes https://bugs.llvm.org/show_bug.cgi?id=39908.

The evaluateGEPOffsetExpression() function simplifies GEP offsets for
use in comparisons against zero, basically by converting X*Scale+Offset==0
to X+Offset/Scale==0 if Scale divides Offset. However, before this is done,
Offset is masked down to the pointer size. This results in incorrect
results for negative Offsets, because we basically end up dividing the
32-bit offset *zero* extended to 64-bit bits (rather than sign extended).

Fix this by explicitly sign extending the truncated value.

Differential Revision: https://reviews.llvm.org/D55449

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@348987 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -522,11 +522,9 @@ static Value *evaluateGEPOffsetExpression(User *GEP, InstCombiner &IC,
   }
 
   // Otherwise, there is an index.  The computation we will do will be modulo
-  // the pointer size, so get it.
-  uint64_t PtrSizeMask = ~0ULL >> (64-IntPtrWidth);
-
-  Offset &= PtrSizeMask;
-  VariableScale &= PtrSizeMask;
+  // the pointer size.
+  Offset = SignExtend64(Offset, IntPtrWidth);
+  VariableScale = SignExtend64(VariableScale, IntPtrWidth);
 
   // To do this transformation, any constant index must be a multiple of the
   // variable scale factor.  For example, we can evaluate "12 + 4*i" as "3 + i",

test/Transforms/InstCombine/pr39908.ll

@@ -0,0 +1,49 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+target datalayout = "p:32:32"
+
+%S = type { [2 x i32] }
+
+define i1 @test([0 x %S]* %p, i32 %n) {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[N:%.*]], 1
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %start.cast = bitcast [0 x %S]* %p to %S*
+  %end = getelementptr inbounds [0 x %S], [0 x %S]* %p, i32 0, i32 %n, i32 0, i32 0
+  %end.cast = bitcast i32* %end to %S*
+  %last = getelementptr inbounds %S, %S* %end.cast, i32 -1
+  %cmp = icmp eq %S* %last, %start.cast
+  ret i1 %cmp
+}
+
+; Same test using 64-bit indices.
+define i1 @test64([0 x %S]* %p, i64 %n) {
+; CHECK-LABEL: @test64(
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc i64 [[N:%.*]] to i32
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[TMP1]], 1
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %start.cast = bitcast [0 x %S]* %p to %S*
+  %end = getelementptr inbounds [0 x %S], [0 x %S]* %p, i64 0, i64 %n, i32 0, i64 0
+  %end.cast = bitcast i32* %end to %S*
+  %last = getelementptr inbounds %S, %S* %end.cast, i64 -1
+  %cmp = icmp eq %S* %last, %start.cast
+  ret i1 %cmp
+}
+
+; Here the offset overflows and is treated modulo 2^32. This is UB.
+define i1 @test64_overflow([0 x %S]* %p, i64 %n) {
+; CHECK-LABEL: @test64_overflow(
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc i64 [[N:%.*]] to i32
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[TMP1]], 1
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %start.cast = bitcast [0 x %S]* %p to %S*
+  %end = getelementptr inbounds [0 x %S], [0 x %S]* %p, i64 0, i64 %n, i32 0, i64 8589934592
+  %end.cast = bitcast i32* %end to %S*
+  %last = getelementptr inbounds %S, %S* %end.cast, i64 -1
+  %cmp = icmp eq %S* %last, %start.cast
+  ret i1 %cmp
+}