[DebugInfo] Add a DW_OP_LLVM_entry_value operation

Summary:
Internally in LLVM's metadata we use DW_OP_entry_value operations with
the same semantics as DWARF; that is, its operand specifies the number
of bytes that the entry value covers.

At the time of emitting entry values we don't know the emitted size of
the DWARF expression that the entry value will cover. Currently the size
is hardcoded to 1 in DIExpression, and other values causes the verifier
to fail. As the size is 1, that effectively means that we can only have
valid entry values for registers that can be encoded in one byte, which
are the registers with DWARF numbers 0 to 31 (as they can be encoded as
single-byte DW_OP_reg0..DW_OP_reg31 rather than a multi-byte
DW_OP_regx). It is a bit confusing, but it seems like llvm-dwarfdump
will print an operation "correctly", even if the byte size is less than
that, which may make it seem that we emit correct DWARF for registers
with DWARF numbers > 31. If you instead use readelf for such cases, it
will interpret the number of specified bytes as a DWARF expression. This
seems like a limitation in llvm-dwarfdump.

As suggested in D66746, a way forward would be to add an internal
variant of DW_OP_entry_value, DW_OP_LLVM_entry_value, whose operand
instead specifies the number of operations that the entry value covers,
and we then translate that into the byte size at the time of emission.

In this patch that internal operation is added. This patch keeps the
limitation that a entry value can only be applied to simple register
locations, but it will fix the issue with the size operand being
incorrect for DWARF numbers > 31.

Reviewers: aprantl, vsk, djtodoro, NikolaPrica

Reviewed By: aprantl

Subscribers: jyknight, fedor.sergeev, hiraditya, llvm-commits

Tags: #debug-info, #llvm

Differential Revision: https://reviews.llvm.org/D67492

llvm-svn: 374881

1 files changed, 91 insertions, 0 deletions

diff --git a/llvm/test/DebugInfo/ARM/entry-value-multi-byte-expr.ll b/llvm/test/DebugInfo/ARM/entry-value-multi-byte-expr.ll
new file mode 100644
index 00000000000..6c482754cda
--- /dev/null
+++ b/llvm/test/DebugInfo/ARM/entry-value-multi-byte-expr.ll
@@ -0,0 +1,91 @@
+; RUN: llc -debug-entry-values -filetype=asm -o - %s | FileCheck %s
+
+; Verify that the size operands of the DW_OP_GNU_entry_value operations are
+; correct for the multi-byte DW_OP_regx expressions.
+
+; Based on the following C reproducer:
+;
+; extern void clobber();
+; double global;
+; int f(double a, double b) {
+;   global = a + b;
+;   clobber();
+;   return 1;
+; }
+
+; This test checks the assembly output rather than the output from
+; llvm-dwarfdump, as the latter printed the DW_OP_regx D0 correctly, even
+; though the entry value's size operand did not fully cover that:
+
+; DW_OP_GNU_entry_value(DW_OP_regx D0), DW_OP_stack_value
+;
+; whereas readelf interpreted it as an DW_OP_GNU_entry_value covering one byte,
+; resulting in garbage data:
+;
+; DW_OP_GNU_entry_value: (DW_OP_regx: 0 (r0)); DW_OP_breg16 (r16): 2; DW_OP_stack_value
+
+; CHECK:      .byte   243       @ DW_OP_GNU_entry_value
+; CHECK-NEXT: .byte   3         @ 3
+; CHECK-NEXT: .byte   144       @ DW_OP_regx
+; CHECK-NEXT: .byte   128       @ 256
+; CHECK-NEXT: .byte   2         @
+; CHECK-NEXT: .byte   159       @ DW_OP_stack_value
+
+; CHECK:      .byte   243       @ DW_OP_GNU_entry_value
+; CHECK-NEXT: .byte   3         @ 3
+; CHECK-NEXT: .byte   144       @ DW_OP_regx
+; CHECK-NEXT: .byte   129       @ 257
+; CHECK-NEXT: .byte   2         @
+; CHECK-NEXT: .byte   159       @ DW_OP_stack_value
+
+target datalayout = "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "armv7-unknown-unknown"
+
+@global = common global double 0.000000e+00, align 8, !dbg !0
+
+; Function Attrs: nounwind
+define arm_aapcs_vfpcc i32 @f(double %a, double %b) #0 !dbg !12 {
+entry:
+  call void @llvm.dbg.value(metadata double %a, metadata !17, metadata !DIExpression()), !dbg !19
+  call void @llvm.dbg.value(metadata double %b, metadata !18, metadata !DIExpression()), !dbg !19
+  %add = fadd double %a, %b, !dbg !20
+  store double %add, double* @global, align 8, !dbg !20
+  tail call arm_aapcs_vfpcc void @clobber(), !dbg !21
+  ret i32 1, !dbg !22
+}
+
+declare arm_aapcs_vfpcc void @clobber()
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+attributes #0 = { nounwind }
+attributes #1 = { nounwind readnone speculatable willreturn }
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!7, !8, !9, !10}
+!llvm.ident = !{!11}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "global", scope: !2, file: !3, line: 4, type: !6, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 10.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, nameTableKind: None)
+!3 = !DIFile(filename: "entry-value-multi-byte-expr.c", directory: "/")
+!4 = !{}
+!5 = !{!0}
+!6 = !DIBasicType(name: "double", size: 64, encoding: DW_ATE_float)
+!7 = !{i32 2, !"Dwarf Version", i32 4}
+!8 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !{i32 1, !"wchar_size", i32 4}
+!10 = !{i32 1, !"min_enum_size", i32 4}
+!11 = !{!"clang version 10.0.0"}
+!12 = distinct !DISubprogram(name: "f", scope: !3, file: !3, line: 6, type: !13, scopeLine: 6, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !16)
+!13 = !DISubroutineType(types: !14)
+!14 = !{!15, !6, !6}
+!15 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!16 = !{!17, !18}
+!17 = !DILocalVariable(name: "a", arg: 1, scope: !12, file: !3, line: 6, type: !6, flags: DIFlagArgumentNotModified)
+!18 = !DILocalVariable(name: "b", arg: 2, scope: !12, file: !3, line: 6, type: !6, flags: DIFlagArgumentNotModified)
+!19 = !DILocation(line: 0, scope: !12)
+!20 = !DILocation(line: 7, scope: !12)
+!21 = !DILocation(line: 8, scope: !12)
+!22 = !DILocation(line: 9, scope: !12)