Re: Clang optimiser vs preproc.c

Thomas Munro <thomas(dot)munro(at)gmail(dot)com> writes:
> On Sat, Dec 16, 2023 at 3:44 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Thomas Munro <thomas(dot)munro(at)gmail(dot)com> writes:
>>> FYI, it looks like there is a big jump in CPU time to compile preproc.c at -O2:
>>> clang15: ~16s
>>> clang16: ~211s
>>> clang17: ~233s

>> What are the numbers for gram.c?

> clang15: ~3.8s
> clang16: ~3.2s
> clang17: ~2.9s

Huh. There's not that many more productions in the ecpg grammar
than the core, so it doesn't seem likely that this is purely a
size-of-file issue. I'd bet on there being something that clang
doesn't do well about the (very stylized) C code being generated
within the grammar productions.

We actually noticed this or a closely-related problem before [1]
and briefly discussed the possibility of rearranging the generated
code to make it less indigestible to clang. But there was no concrete
idea about what to do specifically, and the thread slid off the radar
screen.

regards, tom lane

[1] https://www.postgresql.org/message-id/20220902174033.wgt6li7oiaxmwftn@awork3.anarazel.de