[flang-commits] [flang] 35efd5d - [flang][debug] Don't ask for a name table with -gline-directives-only (#218402)

[via flang-commits]

Author: Abid Qadeer
Date: 2026-08-24T16:55:56+01:00
New Revision: 35efd5d809c96281f3ffbb8513cbeb01df20c79a

URL: https://github.com/llvm/llvm-project/commit/35efd5d809c96281f3ffbb8513cbeb01df20c79a
DIFF: https://github.com/llvm/llvm-project/commit/35efd5d809c96281f3ffbb8513cbeb01df20c79a.diff

LOG: [flang][debug] Don't ask for a name table with -gline-directives-only (#218402)

On current main (since #217132), a unit built with
`-gline-directives-only` crashes the backend when DWARF 5 is requested:

```console
$ echo 'end program' > test.f90
$ flang -gdwarf-5 -gline-directives-only -S -o /dev/null test.f90
flang: llvm/lib/CodeGen/AsmPrinter/DwarfUnit.h:107:
llvm::MCSymbol *llvm::DwarfUnit::getLabelBegin() const:
Assertion `LabelBegin && "LabelBegin is not initialized"' failed.
```

The version has to be spelled out because flang does not yet default to
DWARF 5. Everything else about the command line is ordinary.

Such a unit emits line directives and no `.debug_info`, so the header of
its compile unit is never written and neither is the label that header
defines. Under DWARF 5 the accelerator table indexes every compile unit
whose `nameTableKind` is `Default` and refers to that label, which is
what trips the assertion.

This patch leaves the name table off for that emission kind, as clang
does. Nothing is lost, as the unit has no `.debug_info` for a name table
to point into.

flang gained `-gline-directives-only` in #217132; before that the driver
rejected the option with a warning, so `DebugDirectivesOnly` never
reached this pass and the combination was unreachable.

Co-authored-by: Cursor <cursoragent at cursor.com>

Added: 
    flang/test/Integration/debug-line-directives-only.f90

Modified: 
    flang/lib/Optimizer/Transforms/AddDebugInfo.cpp
    flang/test/Transforms/debug-line-table.fir

Removed: 
    


################################################################################
diff  --git a/flang/lib/Optimizer/Transforms/AddDebugInfo.cpp b/flang/lib/Optimizer/Transforms/AddDebugInfo.cpp
index fdc7b5e1918e4..6bee203f55c5f 100644
--- a/flang/lib/Optimizer/Transforms/AddDebugInfo.cpp
+++ b/flang/lib/Optimizer/Transforms/AddDebugInfo.cpp
@@ -1037,11 +1037,19 @@ void AddDebugInfoPass::runOnOperation() {
   if (!dwarfDebugFlags.empty())
     producerString += " " + dwarfDebugFlags;
   mlir::StringAttr producer = mlir::StringAttr::get(context, producerString);
+  // A unit that only emits line directives has no .debug_info, so the header
+  // of its compile unit, and with it the label the DWARF 5 accelerator table
+  // points at, is never written. Asking for a name table would have that table
+  // reference a label that does not exist. Clang leaves the name table off for
+  // the same reason.
+  mlir::LLVM::DINameTableKind nameTableKind =
+      debugLevel == mlir::LLVM::DIEmissionKind::DebugDirectivesOnly
+          ? mlir::LLVM::DINameTableKind::None
+          : mlir::LLVM::DINameTableKind::Default;
   mlir::LLVM::DICompileUnitAttr cuAttr = mlir::LLVM::DICompileUnitAttr::get(
       mlir::DistinctAttr::create(mlir::UnitAttr::get(context)),
       llvm::dwarf::getLanguage("DW_LANG_Fortran95"), fileAttr, producer,
-      isOptimized, debugLevel, debugInfoForProfiling,
-      /*nameTableKind=*/mlir::LLVM::DINameTableKind::Default,
+      isOptimized, debugLevel, debugInfoForProfiling, nameTableKind,
       splitDwarfFile.empty() ? mlir::StringAttr()
                              : mlir::StringAttr::get(context, splitDwarfFile));
 

diff  --git a/flang/test/Integration/debug-line-directives-only.f90 b/flang/test/Integration/debug-line-directives-only.f90
new file mode 100644
index 0000000000000..7d49d9f0cf8a5
--- /dev/null
+++ b/flang/test/Integration/debug-line-directives-only.f90
@@ -0,0 +1,10 @@
+! Test that -gline-directives-only leaves the name table off.
+
+! RUN: %flang_fc1 -debug-info-kind=line-directives-only -emit-llvm -o - %s \
+! RUN:   | FileCheck %s
+
+! CHECK: !DICompileUnit({{.*}}emissionKind: DebugDirectivesOnly
+! CHECK-SAME: nameTableKind: None
+
+program test
+end program test

diff  --git a/flang/test/Transforms/debug-line-table.fir b/flang/test/Transforms/debug-line-table.fir
index cd46835662ad5..1c5ac6491d849 100644
--- a/flang/test/Transforms/debug-line-table.fir
+++ b/flang/test/Transforms/debug-line-table.fir
@@ -28,7 +28,7 @@ module {
 // FULL: #di_compile_unit = #llvm.di_compile_unit<id = distinct[{{.*}}]<>, sourceLanguage = DW_LANG_Fortran95, file = #di_file, producer = "{{.*}}flang{{.*}}", emissionKind = Full>
 // OPT: #di_compile_unit = #llvm.di_compile_unit<id = distinct[{{.*}}]<>, sourceLanguage = DW_LANG_Fortran95, file = #di_file, producer = "{{.*}}flang{{.*}}", isOptimized = true, emissionKind = Full>
 // LINETABLE: #di_compile_unit = #llvm.di_compile_unit<id = distinct[{{.*}}]<>, sourceLanguage = DW_LANG_Fortran95, file = #di_file, producer = "{{.*}}flang{{.*}}", emissionKind = LineTablesOnly>
-// DIRECTIVES: #di_compile_unit = #llvm.di_compile_unit<id = distinct[{{.*}}]<>, sourceLanguage = DW_LANG_Fortran95, file = #di_file, producer = "{{.*}}flang{{.*}}", emissionKind = DebugDirectivesOnly>
+// DIRECTIVES: #di_compile_unit = #llvm.di_compile_unit<id = distinct[{{.*}}]<>, sourceLanguage = DW_LANG_Fortran95, file = #di_file, producer = "{{.*}}flang{{.*}}", emissionKind = DebugDirectivesOnly, nameTableKind = None>
 // CHECK: #di_subroutine_type = #llvm.di_subroutine_type<callingConvention = DW_CC_normal, types = #di_basic_type, #di_basic_type>
 // CHECK: #[[SB_SUBPROGRAM:.*]] = #llvm.di_subprogram<id = distinct[{{.*}}]<>, compileUnit = #di_compile_unit, scope = #di_file, name = "[[SB_NAME]]", linkageName = "[[SB_NAME]]", file = #di_file, line = 1, scopeLine = 1, subprogramFlags = "Definition|Optimized", type = #di_subroutine_type>
 // CHECK: #[[DECL_SUBPROGRAM:.*]] = #llvm.di_subprogram<scope = #di_file, name = "[[DECL_NAME]]", linkageName = "[[DECL_NAME]]", file = #di_file, line = 1, scopeLine = 1, subprogramFlags = Optimized, type = #di_subroutine_type>