[OpaquePtr][ValueTracking] Check GEP source element type in isPointerOffset()

[aeubanks]

Summary

Fixes a MemCpyOpt miscompile with opaque pointers.
This function can be further cleaned up, but let's just fix the miscompile first.

[aeubanks]

rebase

[nikic]

LGTM

llvm/lib/Analysis/ValueTracking.cpp

if (Offset)
       return Offset;
   }
 
   // Right now we handle the case when Ptr1/Ptr2 are both GEPs with an identical
   // base.  After that base, they may have some number of common (and
   // potentially variable) indices.  After that they handle some constant
   // offset, which determines their offset from each other.  At this point, we
   // handle no other case.
-  if (!GEP1 || !GEP2 || GEP1->getOperand(0) != GEP2->getOperand(0))
+  if (!GEP1 || !GEP2 || GEP1->getOperand(0) != GEP2->getOperand(0) ||
+      GEP1->getSourceElementType() != GEP2->getSourceElementType())
     return None;
 
   // Skip any common indices and track the GEP types.
   unsigned Idx = 1;
   for (; Idx != GEP1->getNumOperands() && Idx != GEP2->getNumOperands(); ++Idx)
     if (GEP1->getOperand(Idx) != GEP2->getOperand(Idx))
       break;
 
   auto Offset1 = getOffsetFromIndex(GEP1, Idx, DL);
   auto Offset2 = getOffsetFromIndex(GEP2, Idx, DL);
   if (!Offset1 || !Offset2)
     return None;
   return *Offset2 - *Offset1;
 }

llvm/test/Transforms/MemCpyOpt/opaque-ptr.ll

 }
 
 %a = type { i64, i64, i64 }
 %b = type { i32, i32, i32 }
 
 define void @test_different_gep_source_elements(ptr %src) {
 ; CHECK-LABEL: @test_different_gep_source_elements(
 ; CHECK-NEXT:    [[PB:%.*]] = getelementptr [[B:%.*]], ptr [[SRC:%.*]], i64 0, i32 1
+; CHECK-NEXT:    store i64 0, ptr [[PB]], align 4
 ; CHECK-NEXT:    [[PA:%.*]] = getelementptr [[A:%.*]], ptr [[SRC]], i64 0, i32 1
 ; CHECK-NEXT:    [[PA2:%.*]] = getelementptr [[A]], ptr [[SRC]], i64 0, i32 2
 ; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr align 4 [[PA]], i8 0, i64 16, i1 false)
 ; CHECK-NEXT:    ret void
 ;
   %pb = getelementptr %b, ptr %src, i64 0, i32 1
   store i64 0, ptr %pb
   %pa = getelementptr %a, ptr %src, i64 0, i32 1
   store i64 0, ptr %pa
   %pa2 = getelementptr %a, ptr %src, i64 0, i32 2
   store i64 0, ptr %pa2
   ret void
 }
 
 declare void @llvm.memset.p0.i64(ptr nocapture, i8, i64, i1)
 declare void @llvm.memcpy.p0.p0.i64(ptr nocapture, ptr nocapture readonly, i64, i1)