[flang][debug] Use DICompileUnit as scope.

[abidh]

This PR is required for #96316 . For assumed size arrays, we don't have the size of the array in the last dimension. This is represented in metadata by having a DISubrange with no entry for count or upper bound.

LLVM verifier allows a DISubrange with no count or upper bound only for Fortran here. This language is set here when DICompileUnit is processed. If a type is processed before DICompileUnit then Verifier will not allow DISubrange without count or upper bound entry. It will fail with error Subrange must contain count or upperBound.

Above scenario easily happens if a subroutine at global scope takes assumed size parameter. The order of operands in DISubprogram is such that Type is processed before Unit so we hit Verifier::visitDISubrange before language has been set to Fortran.

I have worked around this issue by setting the scope of such functions to be DICompileUnit instead of DIFile. As Scope metadata in DISubprogram is processed before Type so this problem goes away.

Some other observations about this change:

1. It does not make any difference to the generated DWARF even with an include file.
2. It bring subroutine inline with module as modules already set their scope to DICompileUnit.
3. I also looked at the classic flang compiler and it also uses DICompileUnit as scope for such subroutines probably for the same reasons.

[llvmbot]

@llvm/pr-subscribers-flang-fir-hlfir

Author: Abid Qadeer (abidh)

Changes

This PR is required for #96316 . For assumed size arrays, we don't have the size of the array in the last dimension. This is represented in metadata by having a DISubrange with no entry for count or upper bound.

LLVM verifier allows a DISubrange with no count or upper bound only for Fortran here. This language is set here when DICompileUnit is processed. If a type is processed before DICompileUnit then Verifier will not allow DISubrange without count or upper bound entry. It will fail with error Subrange must contain count or upperBound.

Above scenario easily happens if a subroutine at global scope takes assumed size parameter. The order of operands in DISubprogram is such that Type is processed before Unit so we hit Verifier::visitDISubrange before language has been set to Fortran.

I have worked around this issue by setting the scope of such functions to be DICompileUnit instead of DIFile. As Scope metadata in DISubprogram is processed before Type so this problem goes away.

Some other observations about this change:

1. It does not make any difference to the generated DWARF even with an include file.
2. It bring subroutine inline with module as modules already set their scope to DICompileUnit.
3. I also looked at the classic flang compiler and it also uses DICompileUnit as scope for such subroutines probably for the same reasons.

---

Full diff: https://github.com/llvm/llvm-project/pull/96345.diff

2 Files Affected:

• (modified) flang/lib/Optimizer/Transforms/AddDebugInfo.cpp (+1-1)
• (modified) flang/test/Transforms/debug-line-table-inc-file.fir (+2-2)

diff --git a/flang/lib/Optimizer/Transforms/AddDebugInfo.cpp b/flang/lib/Optimizer/Transforms/AddDebugInfo.cpp
index 10c71d3fc9551..2216157f3378d 100644
--- a/flang/lib/Optimizer/Transforms/AddDebugInfo.cpp
+++ b/flang/lib/Optimizer/Transforms/AddDebugInfo.cpp
@@ -266,7 +266,7 @@ void AddDebugInfoPass::runOnOperation() {
 
     // Only definitions need a distinct identifier and a compilation unit.
     mlir::DistinctAttr id;
-    mlir::LLVM::DIScopeAttr Scope = fileAttr;
+    mlir::LLVM::DIScopeAttr Scope = cuAttr;
     mlir::LLVM::DICompileUnitAttr compilationUnit;
     mlir::LLVM::DISubprogramFlags subprogramFlags =
         mlir::LLVM::DISubprogramFlags{};
diff --git a/flang/test/Transforms/debug-line-table-inc-file.fir b/flang/test/Transforms/debug-line-table-inc-file.fir
index 065039b59c5ae..dcd11f75a487a 100644
--- a/flang/test/Transforms/debug-line-table-inc-file.fir
+++ b/flang/test/Transforms/debug-line-table-inc-file.fir
@@ -31,7 +31,7 @@ module attributes {dlti.dl_spec = #dlti.dl_spec<>} {
 // CHECK: #[[LOC_INC_FILE:.*]] = loc("{{.*}}inc.f90":1:1)
 // CHECK: #[[LOC_FILE:.*]] = loc("{{.*}}simple.f90":3:1)
 // CHECK: #[[DI_CU:.*]] = #llvm.di_compile_unit<id = distinct[{{.*}}]<>, sourceLanguage = DW_LANG_Fortran95, file = #[[DI_FILE]], producer = "{{.*}}flang{{.*}}", isOptimized = false, emissionKind = LineTablesOnly>
-// CHECK: #[[DI_SP_INC:.*]] = #llvm.di_subprogram<id = distinct[{{.*}}]<>, compileUnit = #[[DI_CU]], scope = #[[DI_FILE]], name = "sinc", linkageName = "_QPsinc", file = #[[DI_INC_FILE]], {{.*}}>
-// CHECK: #[[DI_SP:.*]] = #llvm.di_subprogram<id = distinct[{{.*}}]<>, compileUnit = #[[DI_CU]], scope = #[[DI_FILE]], name = "_QQmain", linkageName = "_QQmain", file = #[[DI_FILE]], {{.*}}>
+// CHECK: #[[DI_SP_INC:.*]] = #llvm.di_subprogram<id = distinct[{{.*}}]<>, compileUnit = #[[DI_CU]], scope = #[[DI_CU]], name = "sinc", linkageName = "_QPsinc", file = #[[DI_INC_FILE]], {{.*}}>
+// CHECK: #[[DI_SP:.*]] = #llvm.di_subprogram<id = distinct[{{.*}}]<>, compileUnit = #[[DI_CU]], scope = #[[DI_CU]], name = "_QQmain", linkageName = "_QQmain", file = #[[DI_FILE]], {{.*}}>
 // CHECK: #[[FUSED_LOC_INC_FILE]] = loc(fused<#[[DI_SP_INC]]>[#[[LOC_INC_FILE]]])
 // CHECK: #[[FUSED_LOC_FILE]] = loc(fused<#[[DI_SP]]>[#[[LOC_FILE]]])

[tblah]

Your reasoning for this change sounds good to me, but I am no expert on DWARF.

+0.5 (hopefully someone with more DWARF experience can review)

[abidh]

Your reasoning for this change sounds good to me, but I am no expert on DWARF.

+0.5 (hopefully someone with more DWARF experience can review)

Thanks for the review. I have now opened #96474 to fix it properly in llvm. In case that fix is accepted, we will not need this work around. So I will put this on hold for now.