[FastISel][DebugInfo] Handle dbg.value targeting allocas

FastISel currently drops dbg.values targeting allocas. It may seem surprising
that a simple case would fail to be lowered, but dbg.values targeting allocas
are not common; we usually have dbg.declares doing that, and those are handled
by the common code between FastISel and SelectionDAGISel.

This patch addresses the issue by querying the static alloca map from FuncInfo.
If we have a frame index for it, we create a DBG_VALUE intrinsic from it.

One X86 test now generates more debug information and an extra redundant label
that we had to account for.

Author: felipepiovezan

llvm/lib/CodeGen/SelectionDAG/FastISel.cpp

@@ -1327,6 +1327,14 @@ bool FastISel::selectIntrinsicCall(const IntrinsicInst *II) {
                         << *DI << "\n");
       return true;
     }
+    if (auto SI = FuncInfo.StaticAllocaMap.find(dyn_cast<AllocaInst>(V));
+        SI != FuncInfo.StaticAllocaMap.end()) {
+      MachineOperand FrameIndexOp = MachineOperand::CreateFI(SI->second);
+      bool IsIndirect = false;
+      BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, MIMD.getDL(), II, IsIndirect,
+              FrameIndexOp, Var, Expr);
+      return true;
+    }
     if (Register Reg = lookUpRegForValue(V)) {
       // FIXME: This does not handle register-indirect values at offset 0.
       if (!FuncInfo.MF->useDebugInstrRef()) {

llvm/test/CodeGen/X86/fast-isel-dbg-value-alloca.ll

@@ -0,0 +1,32 @@
+; RUN: llc -fast-isel -fast-isel-abort=1 -mtriple=x86_64-unknown-unknown -stop-after=finalize-isel %s -o - | \
+; RUN:    FileCheck %s
+
+define void @foo(ptr noalias nocapture %arg) !dbg !38 {
+  %k.debug = alloca ptr, align 8
+  store ptr %arg, ptr %k.debug, align 8, !dbg !70
+  call void @llvm.dbg.value(metadata ptr %k.debug, metadata !55, metadata !DIExpression(DW_OP_deref)), !dbg !70
+; CHECK: call void @llvm.dbg.value(metadata ptr %{{.*}}, metadata ![[VAR:.*]], metadata ![[EXPR:.*]])
+; CHECK: DBG_VALUE %stack.0{{.*}}, $noreg, ![[VAR]], ![[EXPR]]
+  ret void, !dbg !70
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.module.flags = !{!6, !7, !8, !9}
+!llvm.dbg.cu = !{!16}
+
+!6 = !{i32 7, !"Dwarf Version", i32 4}
+!7 = !{i32 2, !"Debug Info Version", i32 3}
+!8 = !{i32 1, !"wchar_size", i32 4}
+!9 = !{i32 8, !"PIC Level", i32 2}
+!16 = distinct !DICompileUnit(language: DW_LANG_Swift, file: !17, producer: "blah", isOptimized: false, runtimeVersion: 5, emissionKind: FullDebug, sysroot: "blah", sdk: "blah")
+!17 = !DIFile(filename: "blah", directory: "blah")
+!38 = distinct !DISubprogram(name: "blah", linkageName: "$blah", scope: !17, file: !17, line: 34, type: !39, scopeLine: 34, spFlags: DISPFlagDefinition, unit: !16, retainedNodes: !43)
+!39 = !DISubroutineType(types: !40)
+!40 = !{!41, !41}
+!41 = !DICompositeType(tag: DW_TAG_structure_type, name: "blah")
+!43 = !{!49, !55}
+!49 = !DILocalVariable(name: "x", arg: 1, scope: !38, file: !17, line: 34, type: !41)
+!55 = !DILocalVariable(name: "k", scope: !56, file: !17, line: 36, type: !41)
+!56 = distinct !DILexicalBlock(scope: !38, file: !17, line: 36, column: 9)
+!70 = !DILocation(line: 36, column: 9, scope: !56)

llvm/test/DebugInfo/COFF/lines-bb-start.ll

@@ -90,6 +90,8 @@ return:                                           ; preds = %if.end, %if.then
 ; CHECK:         .cv_loc {{.*}} # t.c:4:5
 ; CHECK:         jmp     LBB{{.*}}
 ; CHECK: LBB2_{{.*}}:                                 # %if.end
+; CHECK-NEXT: L{{.*}}:
+; CHECK-NEXT: DEBUG_VALUE: lea_dbg_value:
 ; CHECK-NEXT:    .cv_loc {{.*}} # t.c:5:3
 ; CHECK:         leal 4(%esp), %[[reg:[^ ]*]]
 ; CHECK:         movl    %[[reg]], (%esp)