[llvm] f6954bf - Improve the effectiveness of BDCE's debug info salvaging

[Adrian Prantl via llvm-commits]

Author: Adrian Prantl
Date: 2021-09-28T10:24:51-07:00
New Revision: f6954bf80472cbfc06e39dac75a4a72120c9bd15

URL: https://github.com/llvm/llvm-project/commit/f6954bf80472cbfc06e39dac75a4a72120c9bd15
DIFF: https://github.com/llvm/llvm-project/commit/f6954bf80472cbfc06e39dac75a4a72120c9bd15.diff

LOG: 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

Added: 
    

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

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Scalar/BDCE.cpp b/llvm/lib/Transforms/Scalar/BDCE.cpp
index c06125788f37e..7ae1666f4cbb0 100644
--- 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 ed1baa56017af..d72b239e31644 100644
--- 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 {