[WebAssembly] Preserve debug frame base information through register coloring

2 fixes:

Register coloring can re-assign virtual registers. When the frame base register
is colored, update the DwarfFrameBase accordingly When the frame base register
is stackified, do not attempt to encode DW_AT_frame_base as a local In the
future we will presumably want to handle this case better but for now we can
emit worse debug info rather than crashing.

Differential Revision: https://reviews.llvm.org/D73581

llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp

@@ -77,8 +77,13 @@ static unsigned getLocalId(DenseMap<unsigned, unsigned> &Reg2Local,
   auto P = Reg2Local.insert(std::make_pair(Reg, CurLocal));
   if (P.second) {
     // Mark the local allocated for the frame base vreg.
-    if (MFI.isFrameBaseVirtual() && Reg == MFI.getFrameBaseVreg())
+    if (MFI.isFrameBaseVirtual() && Reg == MFI.getFrameBaseVreg()) {
+      LLVM_DEBUG({
+        dbgs() << "Allocating local " << CurLocal << "for VReg "
+               << Register::virtReg2Index(Reg) << '\n';
+      });
       MFI.setFrameBaseLocal(CurLocal);
+    }
     ++CurLocal;
   }
   return P.first->second;

llvm/lib/Target/WebAssembly/WebAssemblyFrameLowering.cpp

@@ -266,7 +266,7 @@ WebAssemblyFrameLowering::getDwarfFrameBase(const MachineFunction &MF) const {
   DwarfFrameBase Loc;
   Loc.Kind = DwarfFrameBase::WasmFrameBase;
   const WebAssemblyFunctionInfo &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
-  if (needsSP(MF)) {
+  if (needsSP(MF) && MFI.isFrameBaseVirtual()) {
     unsigned LocalNum = MFI.getFrameBaseLocal();
     Loc.Location.WasmLoc = {WebAssembly::TI_LOCAL_START, LocalNum};
   } else {

llvm/lib/Target/WebAssembly/WebAssemblyMachineFunctionInfo.h

@@ -101,6 +101,7 @@ public:
     assert(FrameBaseVreg != -1U && "Frame base vreg hasn't been set");
     return FrameBaseVreg;
   }
+  void clearFrameBaseVreg() { FrameBaseVreg = -1U; }
   // Return true if the frame base physreg has been replaced by a virtual reg.
   bool isFrameBaseVirtual() const { return FrameBaseVreg != -1U; }
   void setFrameBaseLocal(unsigned Local) { FrameBaseLocal = Local; }

llvm/lib/Target/WebAssembly/WebAssemblyRegColoring.cpp

@@ -157,6 +157,9 @@ bool WebAssemblyRegColoring::runOnMachineFunction(MachineFunction &MF) {
     Changed |= Old != New;
     UsedColors.set(Color);
     Assignments[Color].push_back(LI);
+    // If we reassigned the stack pointer, update the debug frame base info.
+    if (Old != New && MFI.isFrameBaseVirtual() && MFI.getFrameBaseVreg() == Old)
+      MFI.setFrameBaseVreg(New);
     LLVM_DEBUG(dbgs() << "Assigning vreg" << Register::virtReg2Index(LI->reg)
                       << " to vreg" << Register::virtReg2Index(New) << "\n");
   }

llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp

@@ -857,6 +857,12 @@ bool WebAssemblyRegStackify::runOnMachineFunction(MachineFunction &MF) {
                        !TreeWalker.isOnStack(Reg);
         if (CanMove && hasOneUse(Reg, Def, MRI, MDT, LIS)) {
           Insert = moveForSingleUse(Reg, Op, Def, MBB, Insert, LIS, MFI, MRI);
+
+          // If we are removing the frame base reg completely, remove the debug
+          // info as well.
+          // TODO: Encode this properly as a stackified value.
+          if (MFI.isFrameBaseVirtual() && MFI.getFrameBaseVreg() == Reg)
+            MFI.clearFrameBaseVreg();
         } else if (shouldRematerialize(*Def, AA, TII)) {
           Insert =
               rematerializeCheapDef(Reg, Op, *Def, MBB, Insert->getIterator(),

llvm/test/CodeGen/WebAssembly/debugtest-opt.ll

@@ -0,0 +1,72 @@
+; RUN: llc < %s -filetype=obj -o - | llvm-dwarfdump -
+
+; To regenerate this file, use approximately the following C code:
+; int* globl;
+; void baz(int arg) {
+;   int locl;
+;   globl = &locl;
+; }
+
+; CHECK: DW_TAG_subprogram
+; CHECK-NEXT:                DW_AT_low_pc
+; CHECK-NEXT:                DW_AT_high_pc
+;; Check that we fall back to the default frame base (the global)
+; CHECK-NEXT:                DW_AT_frame_base	(DW_OP_WASM_location 0x1 +0, DW_OP_stack_value)
+
+; TODO: Find a more-reduced test case for The fix in WebAssemblyRegColoring
+
+; ModuleID = 'debugtest-opt.c'
+source_filename = "debugtest-opt.c"
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32"
+
+@globl = hidden local_unnamed_addr global i32* null, align 4, !dbg !0
+
+; Function Attrs: nounwind writeonly
+define hidden void @baz(i32 %arg) local_unnamed_addr #0 !dbg !12 {
+entry:
+  %locl = alloca i32, align 4
+  call void @llvm.dbg.value(metadata i32 %arg, metadata !16, metadata !DIExpression()), !dbg !18
+  %0 = bitcast i32* %locl to i8*, !dbg !19
+  store i32* %locl, i32** @globl, align 4, !dbg !20, !tbaa !21
+  ret void, !dbg !25
+}
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #2
+
+attributes #0 = { nounwind writeonly "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="none" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="generic" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { argmemonly nounwind willreturn }
+attributes #2 = { nounwind readnone speculatable willreturn }
+attributes #3 = { nounwind }
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!8, !9, !10}
+!llvm.ident = !{!11}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "globl", scope: !2, file: !3, line: 2, type: !6, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 11.0.0 (https://github.com/llvm/llvm-project.git ee80f8bef31e0f98c9a0e1d79dc5f1ff51ed9e3a)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, nameTableKind: None)
+!3 = !DIFile(filename: "debugtest-opt.c", directory: "/s/llvm-upstream")
+!4 = !{}
+!5 = !{!0}
+!6 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !7, size: 32)
+!7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!8 = !{i32 7, !"Dwarf Version", i32 4}
+!9 = !{i32 2, !"Debug Info Version", i32 3}
+!10 = !{i32 1, !"wchar_size", i32 4}
+!11 = !{!"clang version 11.0.0 (https://github.com/llvm/llvm-project.git ee80f8bef31e0f98c9a0e1d79dc5f1ff51ed9e3a)"}
+!12 = distinct !DISubprogram(name: "baz", scope: !3, file: !3, line: 14, type: !13, scopeLine: 14, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !15)
+!13 = !DISubroutineType(types: !14)
+!14 = !{null, !7}
+!15 = !{!16, !17}
+!16 = !DILocalVariable(name: "arg", arg: 1, scope: !12, file: !3, line: 14, type: !7)
+!17 = !DILocalVariable(name: "locl", scope: !12, file: !3, line: 15, type: !7)
+!18 = !DILocation(line: 0, scope: !12)
+!19 = !DILocation(line: 15, column: 3, scope: !12)
+!20 = !DILocation(line: 16, column: 9, scope: !12)
+!21 = !{!22, !22, i64 0}
+!22 = !{!"any pointer", !23, i64 0}
+!23 = !{!"omnipotent char", !24, i64 0}
+!24 = !{!"Simple C/C++ TBAA"}
+!25 = !DILocation(line: 17, column: 1, scope: !12)