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

[nikic]

Summary

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).

The masking code could be fixed to properly replicate sign bits. However, as we are operating on inbounds GEPs here, I would expect that we do not have to care about address space overflows, because these would result in poison anyway. If that understanding is correct, then we can just drop this code entirely, which is what this patch does.

[efriedma]

I think I would rather SignExtend64(X, 32) or something like that... yes, it's effectively the same for well-defined code, but I'd rather explicitly truncate the offset rather than implicitly truncate it in ConstantInt::get.

[nikic]

In D55449#1328633, @efriedma wrote:

I think I would rather SignExtend64(X, 32) or something like that... yes, it's effectively the same for well-defined code, but I'd rather explicitly truncate the offset rather than implicitly truncate it in ConstantInt::get.

Another possibility would be to not apply the simplification if the offsets are too large, i.e. make isIntN(IntPtrWidth, Offset) && isIntN(IntPtrWidth, VariableScale) a precondition for this transform. What do you think?

[efriedma]

Checking for overflow would also be fine, but I think you would want to check overflow on each operation that modifies the offset, as opposed to the final result (which might have overflowed the 64-bit integer anyway).

[nikic]

Use SignExtend64 for a signed truncation.

[nikic]

I went with the SignExtend64 approach you suggested. Looking at the code directly above (for the zero offset case), it even explicitly inserts a truncate into IR, so it seems appropriate to just follow suit.

[efriedma]

LGTM

lib/Transforms/InstCombine/InstCombineCompares.cpp

if (Offset == 0) {
     // we don't need to bother extending: the extension won't affect where the
     // computation crosses zero.
     if (VariableIdx->getType()->getPrimitiveSizeInBits() > IntPtrWidth) {
       VariableIdx = IC.Builder.CreateTrunc(VariableIdx, IntPtrTy);
     }
     return VariableIdx;
   }
 
-  // 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;
-
   // 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",
   // but we can't evaluate "10 + 3*i" in terms of i.  Check that the offset is a
   // multiple of the variable scale.
   int64_t NewOffs = Offset / (int64_t)VariableScale;
   if (Offset != NewOffs*(int64_t)VariableScale)
     return nullptr;
 

test/Transforms/InstCombine/pr39908.ll

+; 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 @foo([0 x %S]* %p, i32 %n) {
+; CHECK-LABEL: @foo(
+; 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
+}