[PATCH] D139675: [WebAssembly] Nullify dangling register DBG_VALUEs

[Heejin Ahn via Phabricator via llvm-commits]

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Register-based `DBG_VALUE` should have been
converted into either local-based or stack-operand-based, so at this
point they don't contain any information.

This CL nullifies them, i.e., turning them info
`DBG_VALUE $noreg, $noreg, ...`, which makes the variable appear as
"optimized out". It is not safe to simply remove these instruction
because at this point, because the fact that there is a `DBG_VALUE` here
means the location this variable resides has changed to somewhere else;
we've lost that information where that was.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D139675

Files:
  llvm/lib/Target/WebAssembly/WebAssemblyDebugFixup.cpp
  llvm/test/DebugInfo/WebAssembly/remove-reg-dbg-value.mir


Index: llvm/test/DebugInfo/WebAssembly/remove-reg-dbg-value.mir
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/WebAssembly/remove-reg-dbg-value.mir
@@ -0,0 +1,50 @@
+# RUN: llc -run-pass wasm-debug-fixup %s -o - | FileCheck %s
+
+# Test if '#DEBUG_VALUE' comments for target indices are printed correctly.
+
+--- |
+  target triple = "wasm32-unknown-unknown"
+
+  define void @test_remove_dangling_reg_dbg_value() !dbg !5 {
+    call void @llvm.dbg.value(metadata i32 0, metadata !9, metadata !DIExpression()), !dbg !10
+    call void @llvm.dbg.value(metadata i32 0, metadata !11, metadata !DIExpression()), !dbg !10
+    ret void
+  }
+
+  declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!2, !3, !4}
+
+  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, emissionKind: FullDebug)
+  !1 = !DIFile(filename: "test.c", directory: "")
+  !2 = !{i32 7, !"Dwarf Version", i32 5}
+  !3 = !{i32 2, !"Debug Info Version", i32 3}
+  !4 = !{i32 1, !"wchar_size", i32 4}
+  !5 = distinct !DISubprogram(name: "test_dbg_value_comment", scope: !1, file: !1, line: 1, type: !6, scopeLine: 1, unit: !0)
+  !6 = !DISubroutineType(types: !7)
+  !7 = !{!8}
+  !8 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+  !9 = !DILocalVariable(name: "var0", scope: !5, file: !1, line: 2, type: !8)
+  !10 = !DILocation(line: 0, scope: !5)
+  !11 = !DILocalVariable(name: "var1", scope: !5, file: !1, line: 2, type: !8)
+...
+
+---
+# CHECK-LABEL: name: test_remove_dangling_reg_dbg_value
+name: test_remove_dangling_reg_dbg_value
+liveins:
+  - { reg: '$arguments' }
+body: |
+  ; CHECK: bb.0:
+  ; CHECK: DBG_VALUE $noreg, $noreg, !9, !DIExpression(), debug-location !10
+  ; CHECK-NEXT: DBG_VALUE target-index(wasm-local) + 1, $noreg, !11, !DIExpression(), debug-location !10
+  ; CHECK-NEXT: RETURN
+  bb.0:
+    liveins: $arguments
+    ; This %3 is a danling register and will turn to $noreg
+    DBG_VALUE %3:i32, $noreg, !9, !DIExpression(), debug-location !10
+    ; This debug info will remain the same, because it's in a local
+    DBG_VALUE target-index(wasm-local) + 1, $noreg, !11, !DIExpression(), debug-location !10
+    RETURN implicit-def dead $arguments
+...
Index: llvm/lib/Target/WebAssembly/WebAssemblyDebugFixup.cpp
===================================================================
--- llvm/lib/Target/WebAssembly/WebAssemblyDebugFixup.cpp
+++ llvm/lib/Target/WebAssembly/WebAssemblyDebugFixup.cpp
@@ -59,6 +59,26 @@
   return new WebAssemblyDebugFixup();
 }
 
+// At this very end of the compilation pipeline, if any DBG_VALUEs with
+// registers remain, it means they are dangling info which we failed to update
+// when their corresponding def instruction was transformed/moved/splitted etc.
+// Because Wasm cannot access values in LLVM virtual registers in the debugger,
+// these dangling DBG_VALUEs in effect kill the effect of any previous DBG_VALUE
+// associated with the variable, which will appear as "optimized out".
+static void nullifyDanglingDebugValues(MachineFunction &MF,
+                                       const TargetInstrInfo *TII) {
+  for (auto &MBB : MF) {
+    for (auto &MI : llvm::make_early_inc_range(MBB)) {
+      if (MI.isDebugValue() && MI.getDebugOperand(0).isReg() &&
+          !MI.isUndefDebugValue()) {
+        LLVM_DEBUG(dbgs() << "Warning: dangling DBG_VALUE nullified: " << MI
+                          << "\n");
+        MI.getDebugOperand(0).setReg(Register());
+      }
+    }
+  }
+}
+
 bool WebAssemblyDebugFixup::runOnMachineFunction(MachineFunction &MF) {
   LLVM_DEBUG(dbgs() << "********** Debug Fixup **********\n"
                        "********** Function: "
@@ -137,5 +157,6 @@
            "WebAssemblyDebugFixup: Stack not empty at end of basic block!");
   }
 
+  nullifyDanglingDebugValues(MF, TII);
   return true;
 }


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