[PATCH] D110568: Improve the effectiveness of BDCE's debug info salvaging

[Adrian Prantl via Phabricator via llvm-commits]

aprantl created this revision.
aprantl added reviewers: djtodoro, rastogishubham.
aprantl added a project: debug-info.
Herald added a subscriber: hiraditya.
aprantl requested review of this revision.
Herald added a project: LLVM.

This patch improves the effectiveness of BDCE's debug info salvaging by processing the instructions in reverse order and delaying dropAllReferences until after debug info salvaging. This allows salvaging of entire chains of deleted instructions!

Previously we would remove all references from an instruction, which would make it impossible to use that instruction to salvage a later instruction in the instruction stream, because its operands were already removed.

See also https://reviews.llvm.org/D110462


https://reviews.llvm.org/D110568

Files:
  llvm/lib/Transforms/Scalar/BDCE.cpp
  llvm/test/Transforms/Util/salvage-debuginfo.ll


Index: llvm/test/Transforms/Util/salvage-debuginfo.ll
===================================================================
--- llvm/test/Transforms/Util/salvage-debuginfo.ll
+++ llvm/test/Transforms/Util/salvage-debuginfo.ll
@@ -1,4 +1,5 @@
 ; RUN: opt -adce %s -S -o - | FileCheck %s
+; RUN: opt -bdce %s -S -o - | FileCheck %s
 target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-apple-macosx"
 define void @f(i32) !dbg !8 {
Index: llvm/lib/Transforms/Scalar/BDCE.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/BDCE.cpp
+++ llvm/lib/Transforms/Scalar/BDCE.cpp
@@ -93,7 +93,7 @@
 static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
   SmallVector<Instruction*, 128> Worklist;
   bool Changed = false;
-  for (Instruction &I : instructions(F)) {
+  for (Instruction &I : llvm::reverse(instructions(F))) {
     // If the instruction has side effects and no non-dbg uses,
     // skip it. This way we avoid computing known bits on an instruction
     // that will not help us.
@@ -108,7 +108,6 @@
          wouldInstructionBeTriviallyDead(&I))) {
       salvageDebugInfo(I);
       Worklist.push_back(&I);
-      I.dropAllReferences();
       Changed = true;
       continue;
     }
@@ -155,6 +154,9 @@
     }
   }
 
+  for (Instruction *&I : Worklist)
+    I->dropAllReferences();
+
   for (Instruction *&I : Worklist) {
     ++NumRemoved;
     I->eraseFromParent();


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