[GlobalOpt] Update debug info when changing CC to Fast

[momo5502]

Changing the CC of local functions to fastcc in GlobalOpt causes the PDB to misalign.
Updating the debug info is requried to reflect the change in the PDB.

This fixes #144301

[llvmbot]

@llvm/pr-subscribers-debuginfo

@llvm/pr-subscribers-llvm-transforms

Author: Maurice Heumann (momo5502)

Changes

Changing the CC of local functions to fastcc in GlobalOpt causes the PDB to misalign.
Updating the debug info is requried to reflect the change in the PDB.

This fixes #144301

---

Full diff: https://github.com/llvm/llvm-project/pull/144303.diff

1 Files Affected:

• (modified) llvm/lib/Transforms/IPO/GlobalOpt.cpp (+18)

diff --git a/llvm/lib/Transforms/IPO/GlobalOpt.cpp b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
index 7db0586386506..b0f1dee415efd 100644
--- a/llvm/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
@@ -1920,6 +1920,14 @@ static void RemovePreallocated(Function *F) {
   }
 }
 
+static unsigned char GetDebugInfoFastCC(const Triple &Triple) {
+  if (Triple.isOSWindows() && Triple.isArch32Bit()) {
+    return llvm::dwarf::DW_CC_BORLAND_msfastcall;
+  }
+
+  return llvm::dwarf::DW_CC_normal;
+}
+
 static bool
 OptimizeFunctions(Module &M,
                   function_ref<TargetLibraryInfo &(Function &)> GetTLI,
@@ -1938,6 +1946,9 @@ OptimizeFunctions(Module &M,
     if (hasOnlyColdCalls(F, GetBFI, ChangeableCCCache))
       AllCallsCold.push_back(&F);
 
+  unsigned char DebugInfoFastCC =
+      GetDebugInfoFastCC(Triple(M.getTargetTriple()));
+
   // Optimize functions.
   for (Function &F : llvm::make_early_inc_range(M)) {
     // Don't perform global opt pass on naked functions; we don't want fast
@@ -2021,6 +2032,13 @@ OptimizeFunctions(Module &M,
       // Fast calling convention.
       F.setCallingConv(CallingConv::Fast);
       ChangeCalleesToFastCall(&F);
+
+      if (F.getSubprogram()) {
+        DISubprogram *SP = F.getSubprogram();
+        auto Temp = SP->getType()->cloneWithCC(DebugInfoFastCC);
+        SP->replaceType(MDNode::replaceWithPermanent(std::move(Temp)));
+      }
+
       ++NumFastCallFns;
       Changed = true;
     }

[dwblaikie]

This seems a bit brittle, if other targets had other calling convention choices - perhaps this logic should go wherever the CC is determined?

[momo5502]

I agree. However, I think the mapping for fastcc is determined by a table definition for each architecture. I feel like integrating this there is not easily feasible.

Do you have a recommendation on how this could be done better?

[dwblaikie]

Could you point to the table definition, we/someone could take a look to see how practical it is.