[flang][debug] set DW_AT_main_subprogram for fortran main function

[tblah]

Requested here #111022 (comment)

[llvmbot]

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

Author: Tom Eccles (tblah)

Changes

Requested here #111022 (comment)

---

Full diff: https://github.com/llvm/llvm-project/pull/111350.diff

2 Files Affected:

• (modified) flang/lib/Optimizer/Transforms/AddDebugInfo.cpp (+5)
• (modified) flang/test/Transforms/debug-fn-info.fir (+1-1)

diff --git a/flang/lib/Optimizer/Transforms/AddDebugInfo.cpp b/flang/lib/Optimizer/Transforms/AddDebugInfo.cpp
index 927e275b360898..bf4de78dbdb213 100644
--- a/flang/lib/Optimizer/Transforms/AddDebugInfo.cpp
+++ b/flang/lib/Optimizer/Transforms/AddDebugInfo.cpp
@@ -267,7 +267,9 @@ void AddDebugInfoPass::handleFuncOp(mlir::func::FuncOp funcOp,
   funcName = mlir::StringAttr::get(context, result.second.name);
 
   // try to use a better function name than _QQmain for the program statement
+  bool isMain = false;
   if (funcName == fir::NameUniquer::doProgramEntry()) {
+    isMain = true;
     mlir::StringAttr bindcName =
         funcOp->getAttrOfType<mlir::StringAttr>(fir::getSymbolAttrName());
     if (bindcName)
@@ -302,6 +304,9 @@ void AddDebugInfoPass::handleFuncOp(mlir::func::FuncOp funcOp,
       mlir::LLVM::DISubprogramFlags{};
   if (isOptimized)
     subprogramFlags = mlir::LLVM::DISubprogramFlags::Optimized;
+  if (isMain)
+    subprogramFlags =
+        subprogramFlags | mlir::LLVM::DISubprogramFlags::MainSubprogram;
   if (!funcOp.isExternal()) {
     // Place holder and final function have to have different IDs, otherwise
     // translation code will reject one of them.
diff --git a/flang/test/Transforms/debug-fn-info.fir b/flang/test/Transforms/debug-fn-info.fir
index eaf88152a21bc6..85cfd13643ec3f 100644
--- a/flang/test/Transforms/debug-fn-info.fir
+++ b/flang/test/Transforms/debug-fn-info.fir
@@ -76,7 +76,7 @@ module attributes {dlti.dl_spec = #dlti.dl_spec<>} {
 // CHECK: #[[TY3:.*]] = #llvm.di_subroutine_type<callingConvention = DW_CC_normal, types = #di_null_type, #[[INT4]]>
 
 // Line numbers should match the number in corresponding loc entry.
-// CHECK: #llvm.di_subprogram<{{.*}}name = "mn", linkageName = "_QQmain", file = {{.*}}, line = 15, scopeLine = 15, subprogramFlags = Definition, type = #[[TY0]]>
+// CHECK: #llvm.di_subprogram<{{.*}}name = "mn", linkageName = "_QQmain", file = {{.*}}, line = 15, scopeLine = 15, subprogramFlags = "Definition|MainSubprogram", type = #[[TY0]]>
 // CHECK: #llvm.di_subprogram<{{.*}}name = "fn1", linkageName = "_QFPfn1", file = {{.*}}, line = 26, scopeLine = 26, subprogramFlags = Definition, type = #[[TY1]]>
 // CHECK: #llvm.di_subprogram<{{.*}}name = "fn2", linkageName = "_QFPfn2", file = {{.*}}, line = 43, scopeLine = 43, subprogramFlags = Definition, type = #[[TY2]]>
 // CHECK: #llvm.di_subprogram<{{.*}}name = "fn3", linkageName = "_QFPfn3", file = {{.*}}, line = 53, scopeLine = 53, subprogramFlags = Definition, type = #[[TY3]]>

[DavidSpickett]

Didn't expect something that quickly :)

What does gdb say for info main before and after the change? (note that this is a new feature, I know at least 115 has it)

[tblah]

For

program example
end program

Both before and after this patch using gdb 15.0.50.

(gdb) info main
example

I'm not sure how gdb found the right name before this patch. I can't see the DW_AT_main_subprogram attribute in objdump -g output before this patch and do after.

[DavidSpickett]

Well I didn't look into GDB's code I'm just going on what experts told me. So there could be a heuristic in there too.

Either way according to https://dwarfstd.org/doc/DWARF4.pdf this is a legitimate use of DW_AT_main_subprogram and it's what LLDB will end up looking for in the future.

[DavidSpickett]

LGTM

[abidh]

I'm not sure how gdb found the right name before this patch. I can't see the DW_AT_main_subprogram attribute in objdump -g output before this patch and do after.

We set DW_CC_program as calling convention for main subprogram which could be used by the GDB in absence of DW_AT_main_subprogram.

[abidh]

LGTM.