EmitFuncArgumentDbgValue: Prefer stack slots over registers for stack arguments

While investigating LLVM PR22316 (http://llvm.org/bugs/show_bug.cgi?id=22316)
I started wondering if it were not always preferable to emit the
initial DBG_VALUEs for stack arguments as FI locations instead of
describing the first register they get copied into. The advantage of
doing this is that the arguments will be available as soon as the
stack is setup. As illustrated by the testcase in the PR, the first
copy of the FI into a register may be sunk by MachineSink.cpp into a
later basic block. By describing the argument on the stack, we nicely
circumvent this problem.

<rdar://problem/19583723>

Differential Revision: https://reviews.llvm.org/D41135

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320758 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index bb2abf2..8f3ffb6 100644 (file)
--- a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -4854,6 +4854,13 @@ bool SelectionDAGBuilder::EmitFuncArgumentDbgValue(
     }
   }
 
+  if (!Op && N.getNode())
+    // Check if frame index is available.
+    if (LoadSDNode *LNode = dyn_cast<LoadSDNode>(N.getNode()))
+      if (FrameIndexSDNode *FINode =
+          dyn_cast<FrameIndexSDNode>(LNode->getBasePtr().getNode()))
+        Op = MachineOperand::CreateFI(FINode->getIndex());
+
   if (!Op) {
     // Check if ValueMap has reg number.
     DenseMap<const Value *, unsigned>::iterator VMI = FuncInfo.ValueMap.find(V);
@@ -4889,13 +4896,6 @@ bool SelectionDAGBuilder::EmitFuncArgumentDbgValue(
     }
   }
 
-  if (!Op && N.getNode())
-    // Check if frame index is available.
-    if (LoadSDNode *LNode = dyn_cast<LoadSDNode>(N.getNode()))
-      if (FrameIndexSDNode *FINode =
-          dyn_cast<FrameIndexSDNode>(LNode->getBasePtr().getNode()))
-        Op = MachineOperand::CreateFI(FINode->getIndex());
-
   if (!Op)
     return false;
 

diff --git a/test/DebugInfo/X86/stack-args.ll b/test/DebugInfo/X86/stack-args.ll
new file mode 100644 (file)
index 0000000..ded7134
--- /dev/null
+++ b/test/DebugInfo/X86/stack-args.ll
@@ -0,0 +1,76 @@
+; RUN: llc -filetype=obj -o - %s | llvm-dwarfdump - | FileCheck %s
+; Generated from:
+; void * __attribute__ (( regparm(2) )) f(void *, void *);
+; void * __attribute__ (( regparm(0) )) g(void *, void *);
+;  
+; void *g(void *t, void *k) {
+;   if (k == (void *)0)
+;     return (void *)0;
+;   return f(t, k);
+; }
+
+; CHECK: DW_TAG_formal_parameter
+; CHECK-NEXT: DW_AT_location   (DW_OP_fbreg +4)
+; CHECK-NEXT: DW_AT_name       ("t")
+; CHECK: DW_TAG_formal_parameter
+; CHECK-NEXT: DW_AT_location   (DW_OP_fbreg +8)
+; CHECK-NEXT: DW_AT_name       ("k")
+
+source_filename = "t.c"
+target datalayout = "e-m:e-p:32:32-f64:32:64-f80:32-n8:16:32-S128"
+target triple = "i386--linux-gnu"
+
+; Function Attrs: nounwind
+define i8* @g(i8* %t, i8* %k) local_unnamed_addr #0 !dbg !10 {
+entry:
+  call void @llvm.dbg.value(metadata i8* %t, metadata !14, metadata !DIExpression()), !dbg !16
+  call void @llvm.dbg.value(metadata i8* %k, metadata !15, metadata !DIExpression()), !dbg !17
+  %cmp = icmp eq i8* %k, null, !dbg !18
+  br i1 %cmp, label %return, label %if.end, !dbg !20
+
+if.end:                                           ; preds = %entry
+  %call = tail call i8* @f(i8* inreg %t, i8* inreg nonnull %k) #3, !dbg !21
+  br label %return, !dbg !22
+
+return:                                           ; preds = %entry, %if.end
+  %retval.0 = phi i8* [ %call, %if.end ], [ null, %entry ]
+  ret i8* %retval.0, !dbg !23
+}
+
+declare i8* @f(i8* inreg, i8* inreg) local_unnamed_addr
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.value(metadata, metadata, metadata) #2
+
+attributes #0 = { nounwind  }
+attributes #2 = { nounwind readnone speculatable }
+attributes #3 = { nounwind }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!5, !6, !7, !8}
+!llvm.ident = !{!9}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 6.0.0  (llvm/trunk 319230)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !3)
+!1 = !DIFile(filename: "t.c", directory: "/")
+!2 = !{}
+!3 = !{!4}
+!4 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: null, size: 32)
+!5 = !{i32 1, !"NumRegisterParameters", i32 0}
+!6 = !{i32 2, !"Dwarf Version", i32 4}
+!7 = !{i32 2, !"Debug Info Version", i32 3}
+!8 = !{i32 1, !"wchar_size", i32 4}
+!9 = !{!"clang version 6.0.0  (llvm/trunk 319230)"}
+!10 = distinct !DISubprogram(name: "g", scope: !1, file: !1, line: 4, type: !11, isLocal: false, isDefinition: true, scopeLine: 4, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !13)
+!11 = !DISubroutineType(types: !12)
+!12 = !{!4, !4, !4}
+!13 = !{!14, !15}
+!14 = !DILocalVariable(name: "t", arg: 1, scope: !10, file: !1, line: 4, type: !4)
+!15 = !DILocalVariable(name: "k", arg: 2, scope: !10, file: !1, line: 4, type: !4)
+!16 = !DILocation(line: 4, column: 15, scope: !10)
+!17 = !DILocation(line: 4, column: 24, scope: !10)
+!18 = !DILocation(line: 5, column: 9, scope: !19)
+!19 = distinct !DILexicalBlock(scope: !10, file: !1, line: 5, column: 7)
+!20 = !DILocation(line: 5, column: 7, scope: !10)
+!21 = !DILocation(line: 7, column: 10, scope: !10)
+!22 = !DILocation(line: 7, column: 3, scope: !10)
+!23 = !DILocation(line: 8, column: 1, scope: !10)