| Issue |
104770
|
| Summary |
Unnecessary return statement generated by EmitFunctionEpiloge in musttail cases.
|
| Labels |
clang,
clang:codegen
|
| Assignees |
kiran-isaac
|
| Reporter |
kiran-isaac
|
When clang::musttail is used, and -O is not set, an extra "function epiloge" return case is generated. This seems to be optimized out at any -O level, and automatially optimized out by the backend. I believe this is the case for all musttail calls, but I have attatched a repro example for conveniance.
Reproduce:
```cpp
int F1(signed short P0);
int F2(signed short P0) {
[[clang::musttail]] return F1(2);
}
```
```bash
clang -S -emit-llvm
```
```
define dso_local noundef i32 @_Z2F2s(i16 noundef signext %P0) #0 {
entry:
%P0.addr = alloca i16, align 2
store i16 %P0, ptr %P0.addr, align 2
%call = musttail call noundef i32 @_Z2F1s(i16 noundef signext 2)
ret i32 %call
0: ; No predecessors!
ret i32 undef
}
```
The two calls to CreateRet are here:
`CodeGenFunction::EmitCall`
```c++
// If this is a musttail call, return immediately. We do not branch to the
// epilogue in this case.
if (IsMustTail) {
for (auto it = EHStack.find(CurrentCleanupScopeDepth); it != EHStack.end();
++it) {
EHCleanupScope *Cleanup = dyn_cast<EHCleanupScope>(&*it);
if (!(Cleanup && Cleanup->getCleanup()->isRedundantBeforeReturn()))
CGM.ErrorUnsupported(MustTailCall, "tail call skipping over cleanups");
}
if (CI->getType()->isVoidTy())
Builder.CreateRetVoid();
else
Builder.CreateRet(CI);
Builder.ClearInsertionPoint();
EnsureInsertPoint();
return GetUndefRValue(RetTy);
}
```
and here
`CodeGenFunction::EmitFunctionEpilog`
```c++
case ABIArgInfo::IndirectAliased:
llvm_unreachable("Invalid ABI kind for return argument");
}
llvm::Instruction *Ret;
if (RV) {
if (CurFuncDecl && CurFuncDecl->hasAttr<CmseNSEntryAttr>()) {
// For certain return types, clear padding bits, as they may reveal
// sensitive information.
// Small struct/union types are passed as integers.
auto *ITy = dyn_cast<llvm::IntegerType>(RV->getType());
if (ITy != nullptr && isa<RecordType>(RetTy.getCanonicalType()))
RV = EmitCMSEClearRecord(RV, ITy, RetTy);
}
EmitReturnValueCheck(RV);
Ret = Builder.CreateRet(RV);
} else {
Ret = Builder.CreateRetVoid();
}
```
I suggest that EmitFunctionEpilog should not be called for a musttail function, as the body definitely already contains a return.
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