[flang-commits] [flang] [flang][debug] Use DICompileUnit as scope. (PR #96345)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

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

Author: Abid Qadeer (abidh)

<details>
<summary>Changes</summary>

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](https://github.com/llvm/llvm-project/blob/af478c82a5f6e42ff5d1e67dcefbf9dd0fee501c/llvm/lib/IR/Verifier.cpp#L1159). This language is set [here](https://github.com/llvm/llvm-project/blob/af478c82a5f6e42ff5d1e67dcefbf9dd0fee501c/llvm/lib/IR/Verifier.cpp#L1415) 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](https://github.com/llvm/llvm-project/blob/bf3e3289d67cb0fe136b0660cac39c24c9f65069/llvm/lib/IR/DebugInfoMetadata.cpp#L1140) 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
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]]])

``````````

</details>


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