Improve the effectiveness of ADCE's debug info salvaging

This patch improves the effectiveness of ADCE'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/D110462

Author: adrian-prantl

llvm/lib/Transforms/Scalar/ADCE.cpp

@@ -538,7 +538,7 @@ bool AggressiveDeadCodeElimination::removeDeadInstructions() {
   // that have no side effects and do not influence the control flow or return
   // value of the function, and may therefore be deleted safely.
   // NOTE: We reuse the Worklist vector here for memory efficiency.
-  for (Instruction &I : instructions(F)) {
+  for (Instruction &I : llvm::reverse(instructions(F))) {
     // Check if the instruction is alive.
     if (isLive(&I))
       continue;
@@ -554,9 +554,11 @@ bool AggressiveDeadCodeElimination::removeDeadInstructions() {
     // Prepare to delete.
     Worklist.push_back(&I);
     salvageDebugInfo(I);
-    I.dropAllReferences();
   }
 
+  for (Instruction *&I : Worklist)
+    I->dropAllReferences();
+
   for (Instruction *&I : Worklist) {
     ++NumRemoved;
     I->eraseFromParent();

llvm/test/Transforms/Util/salvage-debuginfo.ll

@@ -5,8 +5,10 @@ define void @f(i32) !dbg !8 {
 entry:
   %p_x = inttoptr i32 %0 to i8*
   %i_x = ptrtoint i8* %p_x to i32
-  ; CHECK: call void @llvm.dbg.value(metadata i8* undef,
-  ; CHECK-SAME: !DIExpression(DW_OP_LLVM_convert, 64, DW_ATE_unsigned,
+  ; CHECK: call void @llvm.dbg.value(metadata i32 %0,
+  ; CHECK-SAME: !DIExpression(DW_OP_LLVM_convert, 32, DW_ATE_unsigned,
+  ; CHECK-SAME:               DW_OP_LLVM_convert, 64, DW_ATE_unsigned,
+  ; CHECK-SAME:               DW_OP_LLVM_convert, 64, DW_ATE_unsigned,
   ; CHECK-SAME:               DW_OP_LLVM_convert, 32, DW_ATE_unsigned, DW_OP_stack_value))
   call void @llvm.dbg.value(metadata i32 %i_x, metadata !11, metadata !DIExpression()), !dbg !13
   ret void, !dbg !13