https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110953
Bug ID: 110953
Summary: Introducing the "wincall" Calling Convention for GCC
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: unlvsur at live dot com
Target Milestone: ---
I present a novel calling convention named "wincall" designed specifically for
GCC. This convention is accompanied by the [[__gnu__::__wincall__]] attribute
and caters to the latest Intel APX instructions on Windows systems, excluding
Linux, BSD, and similar platforms.
Motivation:
The current Windows calling convention exhibits inefficiencies and introduces
performance bottlenecks to C++ programs. This is particularly evident in
libstdc++ components such as "span" and "string_view":
Reference: std::span is not zero-cost on microsoft abi.std::span is not
zero-cost on microsoft abi.
https://www.reddit.com/r/cpp/comments/p0pkcv/stdspan_is_not_zerocost_on_microsoft_abi
https://developercommunity.visualstudio.com/t/post/10433601
The innovative Herbception mechanism, as proposed in P0709 by Herb Sutter,
necessitates passing std::error using two registers and a carry flag. However,
the existing Windows calling convention only allows returning one register.
The current calling conventions allocate just four registers for parameter
passing. Given that Intel has extended x86_64 registers from 16 to 32 for APX,
this presents an opportune moment to introduce a new calling convention to make
optimal use of these additional registers.
Notably, Windows DLL APIs are labeled with [[__gnu__::__stdcall]],
[[__gnu__::__cdecl__]], or [[__gnu__::__fastcall__]]. Implementing this new
convention will not disrupt code that interfaces with DLLs. Furthermore, MSVC
provides an option to toggle the default calling convention.
Eliminating the requirement for empty objects to occupy a register slot would
substantially ease the burden on C++ programmers.
The Windows ABI already follows a caller-saved approach for passing registers,
thus incorporating more registers for parameter passing should not pose issues.
Objectives:
Minimize the register usage for calls into the [[gnu::fastcall]] convention,
the sole existing calling convention for Windows.
Retain caller-saved registers, consistent with Windows conventions.
Ensure compatibility with the Itanium C++ ABI, without impacting the sysvabi.
Implement the proposed "wincall" convention first and allow Microsoft and Clang
to adopt it subsequently.
Seamlessly integrate with the existing Itanium C++ ABI rule for C++ objects'
return behavior (as currently practiced by GCC, not MSVC).
Guidelines:
Eliminate the necessity for empty objects to claim register slots.
Return the first parameter using the rax register and the second parameter
using the rdx register (similar to the 32-bit x86 convention).
When dealing with structures of 16 bits, split them into two parameters (unless
the object is empty, in which case no registers are used). Objects of lengths
1, 2, 4, 8, 16, 32, or 64 bits employ a single register. A 128-bit object uses
two registers, with the remaining bits passed using the object's address.
Adhere to the Itanium ABI rule for C++ objects' return, consistent with GCC's
practice.
Preserve the caller-saved parameter approach utilized in current Windows
conventions.
floating-point and __m128
stack XMM8 XMM7 XMM6 XMM5 XMM4 XMM3 XMM2 XMM1 XMM0
__m256
stack YMM8 YMM7 YMM6 YMM5 YMM4 YMM3 YMM2 YMM1 YMM0
__m512
stack ZMM8 ZMM7 ZMM6 ZMM5 ZMM4 ZMM3 ZMM2 ZMM1 ZMM0
bool, integer and __uint128_t/__int128_t and std::float128_t
stack R19 R18 R17 R16 R9 R8 RDX RCX
Aggregates (8, 16, 32, or 64 bits. 128 bits split to 2) and __m64
stack R19 R18 R17 R16 R9 R8 RDX RCX
Other aggregates, as pointers
stack R19 R18 R17 R16 R9 R8 RDX RCX
Return values
A scalar return value that can fit into 64 bits, including the __m64 type, is
returned through RAX.
A scalar return value that can fit into 128 bits, is returned through RAX (low)
and RDX (high).
Non-scalar types including floats, doubles, and vector types such as __m128,
__m128i, __m128d are returned in XMM0. The state of unused bits in the value
returned in RAX or XMM0 is undefined.
User-defined types can be returned by value from global functions and static
member functions. To return a user-defined type by value in RAX (RDX for 128
bits), it must have a length of 1, 2, 4, 8, 16, 32, 64 or 128 bits.
The "Herbception" concept involves a structure named std::error:
struct error
{
void * domain;
uintptr_t code;
};
In the context of this convention, std::error is passed using rax for the
"domain" and rdx for the "code." Additionally, a carry flag is employed to
handle exceptions. Herbception triggers an exception when the carry flag is
set.
