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Date: Tue, 16 May 2023 17:32:51 +0800
From: "847567161" <847567161@...com>
To: "musl" <musl@...ts.openwall.com>
Subject: Re：Re： Re: Re: Re: [Patch 1/1] vfprintf: optimize vfprintf performance
> Interesting -- so you're lazily constructing the nl_arg data when you
> first discover the need for it, rather than unconditionally doing the
> first pass to look for it. Do you measure a relevant performance
> difference doing this?
Yes, the benchmark show there are about 30% benefits for commonly used format string.
BM_stdio_printf_s, 160 ns（before opt）, 110 ns （after opt）
static void BM_stdio_printf_s(benchmark::State& state) {
while (state.KeepRunning()) {
char buf[BUFSIZ];
snprintf(buf, sizeof(buf), "this is a more typical error message with detail: %s",
"No such file or directory");
}
}
MUSL_BENCHMARK(BM_stdio_printf_s);
> Stylistically, I like that the patch is minimally invasive and does
> not make complex changes (like what I suggested above) together with
> the intended functional change. I don't really like building up extra
> "special meaning" state like whether nl_arg_ptr is null on top of
> whether f is null to represent what mode the printf_core is operating
> in, but this can/should probably be cleaned up as a separate patch to
> keep the functional change itself simple, like what you've done.
I add nl_arg_filled to indicate whether we have completed the acquisition of parameters.
> On a technical note, some compilers are bad about hoisting large local
> objects out of their scopes when inlining. This results in the large
> floating point buffer from fmt_fp getting allocated at the top of
> printf_core, and presumably causes a recursive call to printf_core to
> double the stack requirements here, which is problematic. I'm not sure
> if we should try to avoid recursion (it should be fairly
> straightforward to do so) or just try to fix the unwanted hoisting to
> begin with (something I've been wanting to do for a while).
I modified buf to be declared when using ranther than at the top of printf_core.
> It looks like these two hunks are almost not needed, since it's not
> allowed to mix positional and non-positional forms.
Thanks， I had changed it in patch.
So musl don't support these format strings, right?
printf("yc test: %*2$d \n", 2, 10);
printf("yc test: %.*2$f \n", 1.123456, 10);
Does "%n$d", "%*n$d" or "%.*n$d" should be used together?
> However, %m does
> not take an argument, but might have width/precision in * form. It'd
> be nice if there were some way to avoid this duplication of logic.
Do you mean "%*m$" or "%.*m$"?
What do you mean "duplication of logic"? I don't get it.
> No check has been made that you haven't already consumed arguments via
> non-positional-form format specifiers. This will make things blow up
> much worse if the caller wrongly mixes them, consuming wrong-type args
> from the wrong positions rather than erroring out (as we do now) or
> trapping (probably ideal). Right now we don't actually catch all forms
> of mixing either, but we do necessarily consume all positional args in
> order before attempting to consume any erroneously-specified
> non-positional ones. Your patch changes this to happen in the order
> things were encountered. So I think, as a first step before even doing
> this patch, we should update the l10n flag to a tri-state thing where
> it can be yes/no/indeterminate, to allow erroring out on any specifier
> that mismatches the existing yes/no.
I remember you mentioned "calling printf with an invalid format
string has undefined behavior" here, so maybe it's also ok.
https://www.openwall.com/lists/musl/2023/05/07/1
Chuang Yin
Download attachment "optimize_printf_core.diff" of type "application/octet-stream" (1970 bytes)
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