[DebugInfo] Handle dbg.assigns in FastISel (#80734)

There are some rare circumstances where dbg.assign intrinsics can reach
FastISel. They are a more specialised kind of dbg.value intrinsic with
more information about the originating alloca. They only occur during
optimisation, but might reach FastISel through always_inlining an
optimised function into an optnone function.

This is a slight problem as it's not safe (for debug-info accuracy) to
ignore any intrinsics, and for RemoveDIs (the intrinsic-replacement
project) it causes a crash through an unhandled switch case. To get
around this, we can just treat the dbg.assign as a dbg.value (it's an
actual subclass) and use the variable location information from the
dbg.value fields. This loses a small amount of debug-info about stack
locations, but is more accurate than just ignoring the intrinsic.

(This has popped up deep in an LTO build of a large codebase while
testing RemoveDIs, I figured it'd be good to fix it for the
intrinsic-form at the same time, just to demonstrate the correct
behaviour).

llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp

@@ -2120,6 +2120,13 @@ bool IRTranslator::translateKnownIntrinsic(const CallInst &CI, Intrinsic::ID ID,
                                                 ListSize, Alignment));
     return true;
   }
+  case Intrinsic::dbg_assign:
+    // A dbg.assign is a dbg.value with more information about stack locations,
+    // typically produced during optimisation of variables with leaked
+    // addresses. We can treat it like a normal dbg_value intrinsic here; to
+    // benefit from the full analysis of stack/SSA locations, GlobalISel would
+    // need to register for and use the AssignmentTrackingAnalysis pass.
+    LLVM_FALLTHROUGH;
   case Intrinsic::dbg_value: {
     // This form of DBG_VALUE is target-independent.
     const DbgValueInst &DI = cast<DbgValueInst>(CI);

llvm/lib/CodeGen/SelectionDAG/FastISel.cpp

@@ -1197,7 +1197,8 @@ void FastISel::handleDbgInfo(const Instruction *II) {
       V = DPV.getVariableLocationOp(0);
 
     bool Res = false;
-    if (DPV.getType() == DPValue::LocationType::Value) {
+    if (DPV.getType() == DPValue::LocationType::Value ||
+        DPV.getType() == DPValue::LocationType::Assign) {
       Res = lowerDbgValue(V, DPV.getExpression(), DPV.getVariable(),
                           DPV.getDebugLoc());
     } else {
@@ -1393,6 +1394,13 @@ bool FastISel::selectIntrinsicCall(const IntrinsicInst *II) {
 
     return true;
   }
+  case Intrinsic::dbg_assign:
+    // A dbg.assign is a dbg.value with more information, typically produced
+    // during optimisation. If one reaches fastisel then something odd has
+    // happened (such as an optimised function being always-inlined into an
+    // optnone function). We will not be using the extra information in the
+    // dbg.assign in that case, just use its dbg.value fields.
+    LLVM_FALLTHROUGH;
   case Intrinsic::dbg_value: {
     // This form of DBG_VALUE is target-independent.
     const DbgValueInst *DI = cast<DbgValueInst>(II);

llvm/test/DebugInfo/X86/dont-drop-dbg-assigns-in-isels.ll

@@ -0,0 +1,46 @@
+; RUN: llc %s -fast-isel -start-after=codegenprepare -stop-before=finalize-isel -o - | FileCheck %s
+; RUN: llc %s -fast-isel -start-after=codegenprepare -stop-before=finalize-isel -o - --try-experimental-debuginfo-iterators | FileCheck %s
+; RUN: llc %s -global-isel -start-after=codegenprepare -stop-before=finalize-isel -o - | FileCheck %s
+; RUN: llc %s -global-isel -start-after=codegenprepare -stop-before=finalize-isel -o - --try-experimental-debuginfo-iterators | FileCheck %s
+
+target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-unknown"
+
+; CHECK: DBG_VALUE
+
+declare void @llvm.dbg.assign(metadata, metadata, metadata, metadata, metadata, metadata)
+
+define dso_local i32 @foo(i32 %a, i32 %b) local_unnamed_addr !dbg !8 {
+entry:
+  call void @llvm.dbg.assign(metadata !DIArgList(i32 %a, i32 %b), metadata !16, metadata !DIExpression(DW_OP_LLVM_arg, 0, DW_OP_LLVM_arg, 1, DW_OP_plus), metadata !21, metadata ptr undef, metadata !DIExpression()), !dbg !17
+  %mul = mul nsw i32 %b, %a, !dbg !18
+  ret i32 %mul, !dbg !18
+}
+
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !19, !6}
+!llvm.ident = !{!7}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 11.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "debug_value_list_selectiondag.cpp", directory: "/")
+!2 = !{}
+!3 = !{i32 2, !"CodeView", i32 1}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 2}
+!6 = !{i32 7, !"PIC Level", i32 2}
+!7 = !{!"clang version 11.0.0"}
+!8 = distinct !DISubprogram(name: "foo", linkageName: "foo", scope: !9, file: !9, line: 1, type: !10, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !13)
+!9 = !DIFile(filename: ".\\debug_value_list.cpp", directory: "/tmp")
+!10 = !DISubroutineType(types: !11)
+!11 = !{!12, !12, !12}
+!12 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!13 = !{!14, !15, !16}
+!14 = !DILocalVariable(name: "b", arg: 2, scope: !8, file: !9, line: 1, type: !12)
+!15 = !DILocalVariable(name: "a", arg: 1, scope: !8, file: !9, line: 1, type: !12)
+!16 = !DILocalVariable(name: "c", scope: !8, file: !9, line: 2, type: !12)
+!17 = !DILocation(line: 0, scope: !8)
+!18 = !DILocation(line: 3, scope: !8)
+!19 = !{i32 7, !"debug-info-assignment-tracking", i1 true}
+!20 = !DILocalVariable(name: "d", scope: !8, file: !9, line: 2, type: !12)
+!21 = distinct !DIAssignID()