[PATCH] D142293: [InstCombine] Add PHINode operands to worklist on instruction erasure

[Luke Lau via Phabricator via llvm-commits]

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If a phi node has one use, and that user has only one use itself, and
that one use is in turn the phi node, then the phi node is erased.
Simultaneously, whenever an instruction is erased, its operands are
added to the worklist as their use counts have changed.

But it's possible that the operand now only has one use and that use is
a phi node: So add the phi node to worklist so the phi node elimination
can occur.

This prevents the need to do lots of cascading iterations over a
function when eliminating phi nodes unlocks more potential phi node
eliminations.

Fixes #50564


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D142293

Files:
  llvm/lib/Transforms/InstCombine/InstCombineInternal.h
  llvm/test/Transforms/InstCombine/phi-elimination-iteration.ll


Index: llvm/test/Transforms/InstCombine/phi-elimination-iteration.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/InstCombine/phi-elimination-iteration.ll
@@ -0,0 +1,21 @@
+; RUN: opt < %s -passes=instcombine -S -debug 2>&1 | FileCheck %s
+define void @f() {
+entry:
+	br label %a
+a:
+	%phi.a = phi i32 [ 0, %entry ], [ %x, %a ]
+	%x = add i32 %phi.a, 1
+	br i1 false, label %a, label %b
+b:
+	%phi.b = phi i32 [ %x, %a ], [ %y, %b ]
+	%y = add i32 %phi.b, 1
+	br label %b
+}
+
+; CHECK: INSTCOMBINE ITERATION #1
+; CHECK: INSTCOMBINE ITERATION #2
+; CHECK-NOT: INSTCOMBINE ITERATION #3
+; CHECK-LABEL: a:
+; CHECK-NEXT:    br i1 false, label %a, label %b
+; CHECK-LABEL: b:
+; CHECK-NEXT:    br label %b
Index: llvm/lib/Transforms/InstCombine/InstCombineInternal.h
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -21,6 +21,7 @@
 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/InstVisitor.h"
+#include "llvm/IR/Instructions.h"
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/IR/Value.h"
 #include "llvm/Support/Debug.h"
@@ -412,8 +413,14 @@
     // Make sure that we reprocess all operands now that we reduced their
     // use counts.
     for (Use &Operand : I.operands())
-      if (auto *Inst = dyn_cast<Instruction>(Operand))
+      if (auto *Inst = dyn_cast<Instruction>(Operand)) {
         Worklist.add(Inst);
+        // If this instruction uses a phi node then we need to process the phi
+        // node, as it may potentially be able to delete it
+        for (Use &OpOp : Inst->operands())
+          if (auto *PN = dyn_cast<PHINode>(OpOp))
+            Worklist.add(PN);
+      }
 
     Worklist.remove(&I);
     I.eraseFromParent();


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