[PATCH] D49270: [InstCombine] Preserve debug value when simplifying cast-of-select

[Vedant Kumar via Phabricator via llvm-commits]

vsk created this revision.
vsk added reviewers: lebedev.ri, spatel, gramanas, aprantl.

InstCombine has a cast transform that matches a cast-of-select:

  Orig = cast (Src = select Cond TV FV)

And tries to replace it with a select which has the cast folded in:

  NewSel = select Cond (cast TV) (cast FV)

The combiner does RAUW(Orig, NewSel), so any debug values for Orig would
survive the transform. But debug values for `Src` would be lost.

This patch teaches InstCombine to replace all debug uses of `Src` with
`NewSel` (taking care of doing any necessary DIExpression rewriting).


https://reviews.llvm.org/D49270

Files:
  lib/Transforms/InstCombine/InstCombineCasts.cpp
  test/Transforms/InstCombine/debuginfo-variables.ll


Index: test/Transforms/InstCombine/debuginfo-variables.ll
===================================================================
--- test/Transforms/InstCombine/debuginfo-variables.ll
+++ test/Transforms/InstCombine/debuginfo-variables.ll
@@ -26,6 +26,17 @@
   ret i64 %c2
 }
 
+define i32 @test_cast_select(i1 %cond) {
+; CHECK-LABEL: @test_cast_select(
+; CHECK-NEXT:  [[sel:%.*]] = select i1 %cond, i32 3, i32 5
+; CHECK-NEXT:  call void @llvm.dbg.value(metadata i32 [[sel]], {{.*}}, metadata !DIExpression())
+; CHECK-NEXT:  call void @llvm.dbg.value(metadata i32 [[sel]], {{.*}}, metadata !DIExpression())
+; CHECK-NEXT:  ret i32 [[sel]]
+  %sel = select i1 %cond, i16 3, i16 5
+  %cast = zext i16 %sel to i32
+  ret i32 %cast
+}
+
 define void @test_or(i64 %A) {
 ; CHECK-LABEL: @test_or(
 ; CHECK-NEXT:  call void @llvm.dbg.value(metadata i64 %A, {{.*}}, metadata !DIExpression(DW_OP_constu, 256, DW_OP_or, DW_OP_stack_value))
Index: lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -282,8 +282,10 @@
     // condition may inhibit other folds and lead to worse codegen.
     auto *Cmp = dyn_cast<CmpInst>(Sel->getCondition());
     if (!Cmp || Cmp->getOperand(0)->getType() != Sel->getType())
-      if (Instruction *NV = FoldOpIntoSelect(CI, Sel))
+      if (Instruction *NV = FoldOpIntoSelect(CI, Sel)) {
+        replaceAllDbgUsesWith(*Sel, *NV, CI, DT);
         return NV;
+      }
   }
 
   // If we are casting a PHI, then fold the cast into the PHI.


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