[PATCH] D126835: Round up zero-sized symbols to 1 byte in `.debug_aranges` (without breaking other logic).

[Patrick Walton via Phabricator via llvm-commits]

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This commit modifies the AsmPrinter to avoid emitting any zero-sized symbols to
the .debug_aranges table, by rounding their size up to 1. Entries with zero
length violate the DWARF 5 spec, which states:

> Each descriptor is a triple consisting of a segment selector, the beginning
> address within that segment of a range of text or data covered by some entry
> owned by the corresponding compilation unit, followed by the non-zero length
> of that range.

In practice, these zero-sized entries produce annoying warnings in lld and
cause GNU binutils to truncate the table when parsing it.

Other parts of LLVM, such as DWARFDebugARanges in the DebugInfo module
(specifically the appendRange method), already avoid emitting zero-sized
symbols to .debug_aranges, but not comprehensively in the AsmPrinter. In fact,
the AsmPrinter does try to avoid emitting such zero-sized symbols when labels
aren't involved, but doesn't when the symbol to emitted is a difference of two
labels; this patch extends that logic to handle the case in which the symbol is
defined via labels.

This is a revised version of diff D126257 <https://reviews.llvm.org/D126257>, which was reverted due to breaking
tests. The now-reverted version of this patch didn't distinguish between
symbols that didn't have their size reported to the DwarfDebug handler and
those that had their size reported to be zero. This new version of the patch
instead restricts the special handling only to the symbols whose size is
definitively known to be zero.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D126835

Files:
  llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
  llvm/test/CodeGen/X86/dwarf-aranges-zero-size.ll


Index: llvm/test/CodeGen/X86/dwarf-aranges-zero-size.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/X86/dwarf-aranges-zero-size.ll
@@ -0,0 +1,23 @@
+; Ensures that the AsmPrinter rounds up zero-sized symbols in `.debug_aranges` to one byte.
+;
+; RUN: llc --generate-arange-section < %s | FileCheck %s
+; CHECK: .section .debug_aranges
+; CHECK: .quad EXAMPLE
+; CHECK-NEXT: .quad 1
+; CHECK: .section
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+ at EXAMPLE = constant <{ [0 x i8] }> zeroinitializer, align 1, !dbg !0
+
+!llvm.module.flags = !{!3}
+!llvm.dbg.cu = !{!4}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "EXAMPLE", linkageName: "EXAMPLE", scope: null, file: null, line: 161, type: !2, isLocal: false, isDefinition: true, align: 1)
+!2 = !DIBasicType(name: "()", encoding: DW_ATE_unsigned)
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = distinct !DICompileUnit(language: DW_LANG_Rust, file: !5, producer: "rustc", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: null, globals: !6)
+!5 = !DIFile(filename: "foo", directory: "")
+!6 = !{!0}
Index: llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
===================================================================
--- llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -3042,13 +3042,18 @@
     for (const ArangeSpan &Span : List) {
       Asm->emitLabelReference(Span.Start, PtrSize);
 
-      // Calculate the size as being from the span start to it's end.
-      if (Span.End) {
+      // Calculate the size as being from the span start to its end.
+      //
+      // If the size is zero, then round it up to one byte. The DWARF
+      // specification requires that entries in this table have nonzero
+      // lengths.
+      auto SizeRef = SymSize.find(Span.Start);
+      if ((SizeRef == SymSize.end() || SizeRef->second != 0) && Span.End) {
         Asm->emitLabelDifference(Span.End, Span.Start, PtrSize);
       } else {
         // For symbols without an end marker (e.g. common), we
         // write a single arange entry containing just that one symbol.
-        uint64_t Size = SymSize[Span.Start];
+        auto Size = SizeRef->second;
         if (Size == 0)
           Size = 1;
 


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