Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -522,11 +522,9 @@
   }
 
   // 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",
Index: test/Transforms/InstCombine/pr39908.ll
===================================================================
--- /dev/null
+++ 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
+}