[llvm] [DWARF] Don't search scope chain to find DISubprogram for prologues (PR #107261)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-debuginfo

Author: Jeremy Morse (jmorse)

<details>
<summary>Changes</summary>

Despite having a DISubprogram to hand, this code calls `getInlinedAtScope` to step through all inlined scopes to the outermost one and retrieve... the same DISubprogram. Seemingly this goes back to fd07a2af23c in 2015 -- I anticipate that back then the metadata layout was radically different, and the code simply hasn't been changed since then as it isn't broken.

Nowadays at least, we can just directly look up the subprogram (and I'd like to refactor this area a little).

---
Full diff: https://github.com/llvm/llvm-project/pull/107261.diff


1 Files Affected:

- (modified) llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp (+2-3) 


``````````diff
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index f111b4aea06f1b..1d57b5e4400dc2 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -2199,11 +2199,10 @@ DebugLoc DwarfDebug::emitInitialLocDirective(const MachineFunction &MF,
 
     // Ensure the compile unit is created if the function is called before
     // beginFunction().
-    (void)getOrCreateDwarfCompileUnit(
-        MF.getFunction().getSubprogram()->getUnit());
+    DISubprogram *SP = MF.getFunction().getSubprogram();
+    (void)getOrCreateDwarfCompileUnit(SP->getUnit());
     // We'd like to list the prologue as "not statements" but GDB behaves
     // poorly if we do that. Revisit this with caution/GDB (7.5+) testing.
-    const DISubprogram *SP = PrologEndLoc->getInlinedAtScope()->getSubprogram();
     ::recordSourceLine(*Asm, SP->getScopeLine(), 0, SP, DWARF2_FLAG_IS_STMT,
                        CUID, getDwarfVersion(), getUnits());
     return PrologEndLoc;

``````````

</details>


https://github.com/llvm/llvm-project/pull/107261