Hello Richard,
I'm compiling for aarch64. Indeed, I was expecting conversion via conditional move or set.
I understand that code such as NGINX HTTP parser is suitable for such conversion. But I was expecting that, for example, this code can benefit of it (ngx_hash is an inline function and is a simple xor operation):
>> if (c) {
>> hash = ngx_hash(0, c);
>> r->lowcase_header[0] = c;
>> i = 1;
>> break;
>> }
Thank for your help and your answers.
Best,
Benjamin Minguez
-----Original Message-----
From: Richard Earnshaw (lists) <Richard.Earnshaw@xxxxxxx>
Sent: Thursday, May 18, 2023 1:02 PM
To: Benjamin Minguez <benjamin.minguez@xxxxxxxxxx>; Kyrylo Tkachov <Kyrylo.Tkachov@xxxxxxx>; gcc-help@xxxxxxxxxxx
Subject: Re: Condition execution optimization with gcc 7.5
On 17/05/2023 09:17, Benjamin Minguez via Gcc-help wrote:
> Hello,
>
> I did add -march=armv8-a (and the others armv8.*-a) to GCC command line, but it looks like the conditional execution optimization, cond_exec_find_if_block function, is never called. I enabled all gcc dumps (-da option) and this function debug message are never printed.
Just to be certain, are you compiling for aarch32 (arm/thumb), or aarch64? The latter does not support conditional execution, except via instructions such as CSEL.
[more comments lower down]
> In parallel, I also try with different version of GCC: 9.5.0 and 11.3.0, and again the I had the same results.
>
> Do you have any idea why the this optimization step is not called?
>
> Thank you in advance for your help.
>
> Best,
> Benjamin Minguez
>
> -----Original Message-----
> From: Benjamin Minguez
> Sent: Wednesday, May 10, 2023 8:43 AM
> To: 'Kyrylo Tkachov' <Kyrylo.Tkachov@xxxxxxx>; gcc-help@xxxxxxxxxxx
> Subject: RE: Condition execution optimization with gcc 7.5
>
> Hi,
>
> Thank for the answer.
>
> I had a look at the wrong function definition, gcc-7.5.0/gcc/target.def:
> DEFHOOK
> (have_conditional_execution,
> "This target hook returns true if the target supports conditional execution.\n\
> This target hook is required only when the target has several different\n\
> modes and they have different conditional execution capability, such as ARM.",
> bool, (void),
> default_have_conditional_execution)
> and find this one, gcc-7.5.0/gcc/targhooks.c:
> bool
> default_have_conditional_execution (void)
> {
> return HAVE_conditional_execution;
> }
> Finally, the macro HAVE_conditional_execution is defined here:
> build-gcc/gcc/insn-config.h,
>
> I will investigate the -march or -mcpu option.
>
> Again, thanks a lot,
>
> Benjamin Minguez
>
> -----Original Message-----
> From: Kyrylo Tkachov <Kyrylo.Tkachov@xxxxxxx>
> Sent: Tuesday, May 9, 2023 11:50 AM
> To: Benjamin Minguez <benjamin.minguez@xxxxxxxxxx>;
> gcc-help@xxxxxxxxxxx
> Subject: RE: Condition execution optimization with gcc 7.5
>
> Hi Benjamin,
>
>> -----Original Message-----
>> From: Gcc-help <gcc-help-bounces+kyrylo.tkachov=arm.com@xxxxxxxxxxx>
>> On Behalf Of Benjamin Minguez via Gcc-help
>> Sent: Tuesday, May 9, 2023 8:54 AM
>> To: gcc-help@xxxxxxxxxxx
>> Subject: Condition execution optimization with gcc 7.5
>>
>> Hello everyone,
>>
>> I'm trying to optimize an application that contains a lot of branches.
>> I'm targeting armv8 processors and I'm using GCC 7.5.0 for compatibility reason.
>
> Of course GCC 7.5 is quite old now but if you're forced to use it...
>
>> As the original application is similar to NGINX, I investigated on
>> NGINX. I'm focusing on the HTTP header parsing. Basically, the
>> algorithm parse byte per byte and based on the value stores some variables.
>> Here is an example, /src/http/ngx_http_parse.c: ngx_http_parse_header_line
>> if (c) {
>> hash = ngx_hash(0, c);
>> r->lowcase_header[0] = c;
>> i = 1;
>> break;
>> }
>>
>> if (ch == '_') {
>> if (allow_underscores) {
>> hash = ngx_hash(0, ch);
>> r->lowcase_header[0] = ch;
>> i = 1;
>>
>> } else {
>> r->invalid_header = 1;
>> }
>>
>> break;
>> }
Your example code isn't complete enough to do a full analysis, but I doubt code like this would generate conditional execution anyway. There are several reasons:
1) It's likely too long once machine instructions are generated
2) There are function calls (ngx_hash) in the body of the conditional blocks (calls cannot be conditionally executed); if they are inlined then see 1) above.
3) you have nested conditions (only the innermost block could be conditionally executed).
4) you wouldn't want to conditionally execute 'if (allow_underscores)'
anyway as it's probably highly predictable as a branch.
R.
>> Also, most of branches are not predictable because it compares against
>> data coming from the network.
>> From these observations, I looked at the conditional execution
>> optimization step in GCC and I found this function that should do the work:
>> cond_exec_find_if_block. And how to customize the decision to use
>> conditional instructions:
>
> ... This relates to the arm port i.e. the 32-bit target in Armv8-a, is that what you're targeting?
> AArch64 has had more tuning work put into it over the years so may do better performance-wise if your processor and environment supports it.
> If you're indeed looking at arm...
>
>> #define MAX_CONDITIONAL_EXECUTE
>> arm_max_conditional_execute ()
>> int
>> arm_max_conditional_execute (void)
>> {
>> return max_insns_skipped;
>> }
>> static int max_insns_skipped = 5;
>>
>> I tried to compile NGNIX in -O2 (that should enable if-conversion2)
>> but I did not noticed any change in the code. I enable GCC debug (-da)
>> and also add some debug in this function and I figure out that
>> targetm.have_conditional_execution is set to false.
>>
>> First, do you how to switch this variable to true. I guess it is an
>> option during the configuration step of GCC.
>
> It's definition on that branch is:
> /* Only thumb1 can't support conditional execution, so return true if
> the target is not thumb1. */
> static bool
> arm_have_conditional_execution (void)
> {
> return !TARGET_THUMB1;
> }
>
> So it looks like you're maybe not setting the right -march or -mcpu option to enable the full armv8-a features?
>
> Thanks,
> Kyrill
>
>> Then, I know that the decision to use conditional execution is based
>> on the extra cost added to compute both branches compare to the cost of a branch.
>> In this specific case, branches are miss predicted and the cost is, indeed, high.
>> Do you think that increasing the max_insns_skipped will be enough to
>> help GCC to use conditional execution?
>>
>> Thank you in advance for your answers.
>>
>> Best,
>> Benjamin Minguez
R.
