Messages in this thread | | | Date | Thu, 24 Mar 2022 16:42:10 +0100 | From | Willy Tarreau <> | Subject | Re: [PATCH v1 04/11] tools/nolibc: x86-64: Use appropriate register constraints if exist |
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On Thu, Mar 24, 2022 at 03:33:57PM +0700, Alviro Iskandar Setiawan wrote: > On Thu, Mar 24, 2022 at 2:57 PM Willy Tarreau <w@1wt.eu> wrote: > > On Thu, Mar 24, 2022 at 02:30:32PM +0700, Ammar Faizi wrote: > > > Use appropriate register constraints if exist. Don't use register > > > variables for all inputs. > > > > > > Register variables with "r" constraint should be used when we need to > > > pass data through a specific register to extended inline assembly that > > > doesn't have a specific register constraint associated with it (anything > > > outside %rax, %rbx, %rcx, %rdx, %rsi, %rdi). > > > > > > It also simplifies the macro definition. > > > > I'm a bit bothered by this one because I went the exact opposite route > > in the early design precisely because I found that the current one was > > simpler. [...] > [...] > > I'd say that if there is any technical benefit in doing this (occasional > > code improvement or better support for older or exotic compilers), I'd say > > "ok go for it", but if it's only a matter of taste, I'm not convinced at > > all and am rather seeing this as a regression. Now if there's rough > > consensus around this approach I'll abide, but then I'd request that other > > archs are adapted as well so that we don't keep a different approach only > > for these two ones. > > I don't see any technical benefit for x86-64, so I don't think there > is a need in doing this. Though I personally prefer to use register > constraints if they exist instead of register variables for everything > (oh yeah, matter of taste since I don't have any technical argument to > say it's better respecting the resulting codegen). The only real issue > is for the syscall6() implementation on i386 as we've been bitten by a > real compiler issue. In short, I am neutral on this change.
Just to be clear, I usually only use register constraints as well but I changed this for the syscalls since they were not sufficient, and found that the mix of the two was really not great to deal with.
Thanks, Willy
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