Re: BUG #18839: ARMv7 builds fail due to missing __crc32cw and similar

Mathew Heard <mat999@gmail.com> writes:
> When the configure test is running its forcing armv8-a+crc+simd (has
> __crc32cb, etc)  rather than testing for the feature against target
> architecture (no __crc32cb, etc). While make is compiling for the target
> architecture armv7ve+simd (no __crc32cb, etc). During the make it is of
> course failing to find __crc32cb etc, they only existed during the
> configure test due to the custom cflags used for that feature test (march
> armv8-a+crc+simd).

No, because when pg_crc32c_armv8.c is compiled, that same -march
setting is used for that one specific file -- trace through the
use of CFLAGS_CRC in the Makefiles.  For instance, on buildfarm
member grison[1] which seems like it should be a pretty close match
for your situation, the configure log shows

checking for __crc32cb, __crc32ch, __crc32cw, and __crc32cd with CFLAGS=... (cached) no
checking for __crc32cb, __crc32ch, __crc32cw, and __crc32cd with CFLAGS=-march=armv8-a+crc+simd... (cached) yes
checking which CRC-32C implementation to use... ARMv8 CRC instructions with runtime check

and then in the build log we find

ccache gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels
-Wmissing-format-attribute-Wimplicit-fallthrough=3 -Wcast-function-type -Wshadow=compatible-local -Wformat-security
-fno-strict-aliasing-fwrapv -fexcess-precision=standard -Wno-format-truncation -Wno-stringop-truncation -g -O2
-march=armv8-a+crc+simd-I../../src/port -DFRONTEND -I../../src/include  -D_GNU_SOURCE -I/usr/include/libxml2   -c -o
pg_crc32c_armv8.opg_crc32c_armv8.c 

(note the -march flag) so that's expected to compile, and does,
even though the platform is armv7l in general.  I have no idea whether
grison's underlying hardware is v7 or v8, and it shouldn't matter for
this purpose, because the only thing at stake is the build toolchain's
behavior.

> I'm not really sure what's so confusing about this situation. It seems like
> a configure feature test that will always detect CRC support on any ARM gcc
> (that supports those functions). I'm mostly surprised your arm build
> environment is passing.

I'm not sure how I can put this any more plainly: it works fine for
everyone but you, including ARMv7 machines.  I have no intention of
regressing performance for everyone who runs on mixed ARMv7/v8
platforms in order to un-break your build.  We need to figure out what
is unusual about your build environment that is causing this mechanism
not to work as-expected.

            regards, tom lane

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=grison&dt=2025-03-14%2000%3A04%3A12