[InstCombine] Call InstSimplify for cast instructions

[nikic]

InstCombine currently fails to call into InstSimplify for cast instructions. I noticed this because the transform from #98649 can be triggered via -passes=instsimplify but not -passes=instcombine, which is not supposed to happen.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Nikita Popov (nikic)

Changes

InstCombine currently fails to call into InstSimplify for cast instructions. I noticed this because the transform from #98649 can be triggered via -passes=instsimplify but not -passes=instcombine, which is not supposed to happen.

---

Full diff: https://github.com/llvm/llvm-project/pull/162849.diff

3 Files Affected:

• (modified) llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp (+3-3)
• (modified) llvm/test/Transforms/InstCombine/cast-set-preserve-signed-dbg-val.ll (+1-3)
• (modified) llvm/test/Transforms/InstCombine/ptr-int-cast.ll (+11)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 4c9b10a094981..cdc559b489e9d 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -156,9 +156,9 @@ Instruction *InstCombinerImpl::commonCastTransforms(CastInst &CI) {
   Value *Src = CI.getOperand(0);
   Type *Ty = CI.getType();
 
-  if (auto *SrcC = dyn_cast<Constant>(Src))
-    if (Constant *Res = ConstantFoldCastOperand(CI.getOpcode(), SrcC, Ty, DL))
-      return replaceInstUsesWith(CI, Res);
+  if (Value *Res =
+          simplifyCastInst(CI.getOpcode(), Src, Ty, SQ.getWithInstruction(&CI)))
+    return replaceInstUsesWith(CI, Res);
 
   // Try to eliminate a cast of a cast.
   if (auto *CSrc = dyn_cast<CastInst>(Src)) {   // A->B->C cast
diff --git a/llvm/test/Transforms/InstCombine/cast-set-preserve-signed-dbg-val.ll b/llvm/test/Transforms/InstCombine/cast-set-preserve-signed-dbg-val.ll
index 7cc4446f1038b..ad45d1e3e3e4f 100644
--- a/llvm/test/Transforms/InstCombine/cast-set-preserve-signed-dbg-val.ll
+++ b/llvm/test/Transforms/InstCombine/cast-set-preserve-signed-dbg-val.ll
@@ -11,10 +11,8 @@ define i16 @test5(i16 %A) !dbg !34 {
   call void @llvm.dbg.value(metadata i32 %C, metadata !37, metadata !DIExpression()), !dbg !41
 
   ; Preserve the dbg.value for the DCE'd 32-bit 'and'.
-  ;
-  ; The high 16 bits of the original 'and' require sign-extending the new 16-bit and:
   ; CHECK-NEXT: #dbg_value(i16 [[and]], [[C:![0-9]+]],
-  ; CHECK-SAME:    !DIExpression(DW_OP_LLVM_convert, 16, DW_ATE_signed, DW_OP_LLVM_convert, 32, DW_ATE_signed, DW_OP_stack_value)
+  ; CHECK-SAME:    !DIExpression(DW_OP_LLVM_convert, 16, DW_ATE_unsigned, DW_OP_LLVM_convert, 32, DW_ATE_unsigned, DW_OP_stack_value)
 
   %D = trunc i32 %C to i16, !dbg !42
   call void @llvm.dbg.value(metadata i16 %D, metadata !38, metadata !DIExpression()), !dbg !42
diff --git a/llvm/test/Transforms/InstCombine/ptr-int-cast.ll b/llvm/test/Transforms/InstCombine/ptr-int-cast.ll
index 69b8f6953d61e..82ecbd41e50a7 100644
--- a/llvm/test/Transforms/InstCombine/ptr-int-cast.ll
+++ b/llvm/test/Transforms/InstCombine/ptr-int-cast.ll
@@ -86,3 +86,14 @@ define <4 x ptr> @test7(<4 x i128> %arg) nounwind {
   %p1 = inttoptr <4 x i128> %arg to <4 x ptr>
   ret <4 x ptr> %p1
 }
+
+define i64 @ptrtoint_gep_sub(ptr %ptr, i64 %end.addr) {
+; CHECK-LABEL: @ptrtoint_gep_sub(
+; CHECK-NEXT:    ret i64 [[END_ADDR:%.*]]
+;
+  %ptr.addr = ptrtoint ptr %ptr to i64
+  %size = sub i64 %end.addr, %ptr.addr
+  %end = getelementptr i8, ptr %ptr, i64 %size
+  %end.addr2 = ptrtoint ptr %end to i64
+  ret i64 %end.addr2
+}

[nikic]

My reading of this DIExpression is that we're first extending to 32-bit and then truncation to 16-bit, in which case the signedness does not matter. But I'm not confident on that reading...

The difference in behavior is that we're directly removing a cast pair instead of converting it into a bitcast first, which has some special handling here, though I have no idea what it's doing: https://github.com/nikic/llvm-project/blob/087ad0bea2e991d6c6b546170994cb2d172a6261/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp#L169-L171