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Re: [PATCH] mips/o32: fix internal_syscall5/6/7
On 2017-08-17 18:09, Adhemerval Zanella wrote:
> On 17/08/2017 17:34, Maciej W. Rozycki wrote:
> > On Thu, 17 Aug 2017, Adhemerval Zanella wrote:
> >> My point is I think we should aim for compiler optimization safeness
> >> (to avoid code breakage over compiler defined default flags) and taking
> >> as base current approach to *avoid* VLA on GLIBC I do not think it is
> >> good approach to use it as a bridge to force GCC to generate the expected
> >> code.
> > You certainly have a point here overall, although I don't think a VLA
> > whose size is always 0 really hurts. And we've used the approach with
> > `alloca' since forever with no adverse effects until we added a place
> > where the caller invokes the syscall wrapper in a loop. So I wouldn't
> > necessarily call it an issue. Mind that this is target-specific code, so
> > we can rely on a target-specific execution model rather than limiting
> > ourselves to what generic ISO C guarantees.
> > Aurelien's figures indicating a clear size reduction certainly count as a
> > pro though.
> Joseph pointed out another advantage of avoid VLAs (building with
> -Werror=alloca -Werror=vla). My main problem here is we are betting that
> compiler won't mess with our assumptions and generate the desirable code
> without trying to adhere what it is suppose to provide. Target generic
> ISO C give us a better guarantee and any deviation indicates a possible
> compiler issue, not otherwise (such this case). My another point is we
> can optimize if required later if this is the case and imho this is hardly
> the case here (at least for latency).
> If I understood correctly Aurelien's suggestion of returning err in v1
> is not ABI strictly so it will end up calling __libc_do_syscall with a
> non-conformant ABI convention (similar to pipe implementation where requires
> assembly specific implementation for a lot of architectures to get this
> right). Again this is something I would really to avoid.
In the ABI v1 is used in pair with v0 to return 64-bit values. In my
patch the __libc_do_syscall is declared as returning a long long. The
value is then split using a union, in a similar way to what is already
done for the mips16 code.
Aurelien Jarno GPG: 4096R/1DDD8C9B