[cfe-dev] CXX ABI, force passing function argument by reference

[Diogo Sampaio via cfe-dev]

For future readers, it can be handled in the target specific <TARGET>ABIInfo::classifyType method. Don't know if this is the best place for it though.

Diogo Sampaio 
Senior Compiler Engineer • Kalray 
dsampaio at kalrayinc.com • [ https://www.kalrayinc.com/ | www.kalrayinc.com ] 

[ https://www.kalrayinc.com/ |   ] 

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----- Original Message -----
From: "cfe-dev" <cfe-dev at lists.llvm.org>
To: "cfe-dev" <cfe-dev at lists.llvm.org>
Sent: Wednesday, November 24, 2021 9:37:38 AM
Subject: [cfe-dev] CXX ABI, force passing function argument by reference

Hi all,

A clang noob question...

In our target C++ ABI aggregates that are not trivially copyable should be passed by reference.
For example, in https://godbolt.org/z/1crdTGeKv

We have the string iii passed by a pointer (from the caller stack main) to function test_it

        addd $r0 = $r12, 64
        call test_it(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)

However our current llvm implementation breaks this abi, passing it by copy in registers:
        ld $r0 = 0[$r12]
        ;;
        ld $r1 = 8[$r12]
        ;;
        ld $r2 = 16[$r12]
        ;;
        ld $r3 = 24[$r12]
        call test_it(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)

Could anyone point me in how to change clang's choice of passing it by copy in register?

I've implemented a target specific CXXABI based in ItaniumCXXABI and overrided getRecordArgABI to return RAA_Indirect or RAA_DirectInMemory if the CXXRecordDecl is not marked as isTriviallyCopyable, and made our target use this CXXABI. However that does not do the trick. Even if I make it always return RAA_DirectInMemory, it does not change a thing in this code.

Thanks in advance,

Diogo Sampaio 
Senior Compiler Engineer • Kalray 
dsampaio at kalrayinc.com • [ https://www.kalrayinc.com/ | www.kalrayinc.com ] 

[ https://www.kalrayinc.com/ |   ] 

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