[cfe-dev] Reading execise

[Hubert Tong via cfe-dev]

On Fri, Sep 18, 2020 at 1:19 PM Stefan Kanthak via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> <
> https://docs.microsoft.com/en-us/cpp/build/x64-calling-convention#return-values
> >
>
> | To return a user-defined type by value in RAX, it must have a length
> | of 1, 2, 4, 8, 16, 32, or 64 bits. [...] Otherwise, the caller must
> | allocate memory for the return value and pass a pointer to it as the
> | first argument. The remaining arguments are then shifted one argument
> | to the right. The same pointer must be returned by the callee in RAX.
>
Is there a particular reason for starting a new thread?

Statements have been made in
https://lists.llvm.org/pipermail/cfe-dev/2020-September/066811.html to
clarify the relationship between __[u]int128_t and the MS ABI.


>
> --- incompatible.c ---
> __attribute__((ms_abi))
> __uint128_t buggy(__uint128_t a, __uint128_t b, __uint128_t c) {
>     return a + b * c;
> }
> --- EOF ---
>
> clang -mno-sse -o- -O3 -S -target amd64-pc-windows incompatible.c
>
> buggy:                                  # @buggy
> # %bb.0:
>         movq    (%rdx), %r9             # OUCH: before it was destroyed,
>                                         #       r9 held the address of c
>
It might help to provide the source code for a caller of this function and
the associated assembly of the caller to demonstrate such a mismatch.


>         movq    (%r8), %rax
>         movq    8(%rdx), %r10
>         imulq   %rax, %r10
>         mulq    %r9                     # OUCH: the source says b * c,
>                                         #       NOT a * <something>
>         imulq   8(%r8), %r9             # OUCH: idem
>         addq    %r10, %rdx
>         addq    %r9, %rdx
>         addq    (%rcx), %rax            # OUCH: rcx holds the address of
>                                         #       the return value!
>                                         # OUCH: rax does NOT point to the
>                                         #       return value!
>         adcq    8(%rcx), %rdx           # OUCH: rcx holds the address of
>                                         #       the return value!
>         retq
>
>
> Stefan
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