[DeadStoreElimination] Salvage debug info from dead insts

[vsk]

Summary

According to llvm-dwarfdump --statistics this salvages 43 additional
unique source variables in a stage2 build of clang. It increases the
size of the .debug_loc section by 0.002% (or 2864 bytes).

[davide]

This is fine, but haven't looked whether DSE has other places where it removes instructions.

[vsk]

In D43220#1006218, @davide wrote:

This is fine, but haven't looked whether DSE has other places where it removes instructions.

Thanks. This is the only site which calls eraseFromParent(), so we should be set.

lib/Transforms/Scalar/DeadStoreElimination.cpp

deleteDeadInstruction(Instruction *I, BasicBlock::iterator *BBI,
   // caller what the next instruction is after we're done mucking about.
   BasicBlock::iterator NewIter = *BBI;
 
   // Before we touch this instruction, remove it from memdep!
   do {
     Instruction *DeadInst = NowDeadInsts.pop_back_val();
     ++NumFastOther;
 
+    // Try to preserve debug information attached to the dead instruction.
+    salvageDebugInfo(*DeadInst);
+
     // This instruction is dead, zap it, in stages.  Start by removing it from
     // MemDep, which needs to know the operands and needs it to be in the
     // function.
     MD.removeInstruction(DeadInst);
 
     for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
       Value *Op = DeadInst->getOperand(op);
       DeadInst->setOperand(op, nullptr);

test/Transforms/DeadStoreElimination/debuginfo.ll

+; RUN: opt < %s -debugify -basicaa -dse -S | FileCheck %s
+
+target datalayout = "E-p:64:64:64-a0:0:8-f32:32:32-f64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-v64:64:64-v128:128:128"
+
+declare noalias i8* @malloc(i32)
+
+declare void @test_f()
+
+define i32* @test_salvage() {
+; CHECK-LABEL: @test_salvage()
+; CHECK-NEXT: malloc
+; CHECK-NEXT: bitcast
+; CHECK-NEXT: call void @test_f()
+; CHECK-NEXT: store i32 0, i32* %P
+
+; Check that all four original local variables have their values preserved.
+; CHECK-NEXT: call void @llvm.dbg.value(metadata i8* %p, metadata !8, metadata !DIExpression()), !dbg !14
+; CHECK-NEXT: call void @llvm.dbg.value(metadata i32* %P, metadata !10, metadata !DIExpression()), !dbg !15
+; CHECK-NEXT: call void @llvm.dbg.value(metadata i32* %P, metadata !11, metadata !DIExpression(DW_OP_deref)), !dbg !18
+; CHECK-NEXT: call void @llvm.dbg.value(metadata i32* %P, metadata !13, metadata !DIExpression(DW_OP_deref, DW_OP_plus_uconst, 1, DW_OP_stack_value)), !dbg !19
+  %p = tail call i8* @malloc(i32 4)
+  %P = bitcast i8* %p to i32*
+  %DEAD = load i32, i32* %P
+  %DEAD2 = add i32 %DEAD, 1
+  store i32 %DEAD2, i32* %P
+  call void @test_f()
+  store i32 0, i32* %P
+  ret i32* %P
+}
+
+; CHECK: !8 = !DILocalVariable(name: "1", scope: !5, file: !1, line: 1, type: !9)
+; CHECK: !10 = !DILocalVariable(name: "2", scope: !5, file: !1, line: 2, type: !9)
+; CHECK: !11 = !DILocalVariable(name: "3", scope: !5, file: !1, line: 3, type: !12)
+; CHECK: !13 = !DILocalVariable(name: "4", scope: !5, file: !1, line: 4, type: !12)
+; CHECK-DAG: !14 = !DILocation(line: 1, column: 1, scope: !5)
+; CHECK-DAG: !15 = !DILocation(line: 2, column: 1, scope: !5)
+; CHECK-DAG: !18 = !DILocation(line: 3, column: 1, scope: !5)
+; CHECK-DAG: !19 = !DILocation(line: 4, column: 1, scope: !5)
+; CHECK-DAG: !16 = !DILocation(line: 6, column: 1, scope: !5)
+; CHECK-DAG: !17 = !DILocation(line: 7, column: 1, scope: !5)