[WIP][IC][DebugInfo] Preserve DI value when folding cast into phi

[gramanas]

Summary

This patch tries to incorporate the new replaceAllDbgUsersWith
function while folding a cast into a phi instruction.

[vsk]

Thanks, a few comments inline --

test/Transforms/InstCombine/phi_cast.ll

7

Why not define in 'i1 %cond' etc. as function parameters? Then the icmp and sdiv become unnecessary.

19

Hold on, if this is a phi i1, then doesn't that mean that 'foldOpIntoPhi' didn't trigger?

24

Doesn't "i" mean "An integer constant (of target-specific width)"? I'm not sure whether the instruction is well-formed without a constraint, but if it isn't, then the "r" (register) constraint seems like the appropriate one here.

[gramanas]

• Update test according to comments.

The result of instcombine doesn't seem to be affected by the asm sideeffect or
by the replaceAllDbgUsersWith though.

[vsk]

In D49677#1172111, @gramanas wrote:

• Update test according to comments.

The result of instcombine doesn't seem to be affected by the asm sideeffect or
by the replaceAllDbgUsersWith though.

Why is the side-effecting use necessary? Have you tried using any other constructions to get to a minimal test?

lib/Transforms/InstCombine/InstCombineCasts.cpp

Instruction *InstCombiner::commonCastTransforms(CastInst &CI) {
   }
 
   // If we are casting a PHI, then fold the cast into the PHI.
   if (auto *PN = dyn_cast<PHINode>(Src)) {
     // Don't do this if it would create a PHI node with an illegal type from a
     // legal type.
     if (!Src->getType()->isIntegerTy() || !CI.getType()->isIntegerTy() ||
         shouldChangeType(CI.getType(), Src->getType()))
-      if (Instruction *NV = foldOpIntoPhi(CI, PN))
+      if (Instruction *NV = foldOpIntoPhi(CI, PN)) {
+        replaceAllDbgUsesWith(*PN, *NV, CI, DT);
         return NV;
       }
+  }
 
   return nullptr;
 }
 
 /// Constants and extensions/truncates from the destination type are always
 /// free to be evaluated in that type. This is a helper for canEvaluate*.
 static bool canAlwaysEvaluateInType(Value *V, Type *Ty) {
   if (isa<Constant>(V))

test/Transforms/InstCombine/phi_cast.ll

+;; RUN: opt < %s -debugify -instcombine -S | FileCheck %s
+
+;; This is a sample test file for doing the testing while
+;; the patch is WIP. Once it's finished it will be incorporated
+;; in the debuginfo-variables.ll file.
+
+define i32 @icmp_div(i16 %a, i16 %c) {
+entry:
+  %tobool = icmp eq i16 %a, 0
+  br i1 %tobool, label %then, label %exit
+
+then:
+  %div = sdiv i16 %c, -1
+  %cmp = icmp ne i16 %div, 0
+  br label %exit
+
+exit:
+;; CHECK-LABEL: exit:
+;; CHECK:         [[phi:%.*]] = phi i1 {{.*}}
+;; CHECK-NEXT:    call void @llvm.dbg.value(metadata i1 [[phi]], {{.*}}, metadata !DIExpression())
+;; CHECK:         [[zext:%.*]] = zext i1 %phi to i32
+;; CHECK-NEXT:    call void @llvm.dbg.value(metadata i32 [[zext]], {{.*}}, metadata !DIExpression())
+  %phi = phi i1 [ false, %entry ], [ %cmp, %then ]
+  call void asm sideeffect "", "i"(i1 %phi)
+  %zext = zext i1 %phi to i32
+  ret i32 %zext
+}