Improve the effectiveness of BDCE's debug info salvaging

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.

Differential Revision: https://reviews.llvm.org/D110568

diff --git a/llvm/lib/Transforms/Scalar/BDCE.cpp b/llvm/lib/Transforms/Scalar/BDCE.cpp
index c061257..7ae1666 100644 (file)
--- a/llvm/lib/Transforms/Scalar/BDCE.cpp
+++ b/llvm/lib/Transforms/Scalar/BDCE.cpp
@@ -93,7 +93,7 @@ static void clearAssumptionsOfUsers(Instruction *I, DemandedBits &DB) {
 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 @@ static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
          wouldInstructionBeTriviallyDead(&I))) {
       salvageDebugInfo(I);
       Worklist.push_back(&I);
-      I.dropAllReferences();
       Changed = true;
       continue;
     }
@@ -155,6 +154,9 @@ static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
     }
   }
 
+  for (Instruction *&I : Worklist)
+    I->dropAllReferences();
+
   for (Instruction *&I : Worklist) {
     ++NumRemoved;
     I->eraseFromParent();

diff --git a/llvm/test/Transforms/Util/salvage-debuginfo.ll b/llvm/test/Transforms/Util/salvage-debuginfo.ll
index ed1baa5..d72b239 100644 (file)
--- a/llvm/test/Transforms/Util/salvage-debuginfo.ll
+++ b/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 {