Unnecessary return statement in unreachable block generated by EmitFunctionEpilog in musttail cases. #104770
Description
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:
int F1(signed short P0); int F2(signed short P0) { [[clang::musttail]] return F1(2); }clang -S -emit-llvmdefine 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
// 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); }Note how the insert point is unset here, as no more code should be generated for this function
and here
CodeGenFunction::EmitFunctionEpilog
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.