[cfe-dev] Clang build of PostgreSQL

[Eli Friedman]

On Thu, Jun 23, 2011 at 2:32 PM, Argyrios Kyrtzidis <kyrtzidis at apple.com> wrote:
> On Jun 23, 2011, at 2:07 PM, Peter Geoghegan wrote:
>
>> On 23 June 2011 21:47, Sean McBride <sean at rogue-research.com> wrote:
>>
>>> Those numbers are for gcc 4.2, and probably Apple's fork thereof.  One would assume the gcc folks have made improvements in 4.6, which you used.
>>
>> I do appreciate that, and that a different optimisation level has been
>> used, but I found the gap in compile times surprising, to the extent
>> that I suspect that something is wrong.
>>
>> Regardless of its performance, I do really like Clang from a usability
>> perspective, FWIW.
>>
>> Observe the differences in compile times of the file
>> /src/backend/parser/gram.c . The warning message that you see is a
>> known bug with Gnu Bison that they refuse to fix:
>>
>> [peter at peter postgresql]$ time /home/peter/build/Release/bin/clang -O2
>> -Wall -Wmissing-prototypes -Wpointer-arith
>> -Wdeclaration-after-statement -Wendif-labels -Wformat-security
>> -fno-strict-aliasing -fwrapv -Wno-error -I. -I. -I
>> /home/peter/postgresql/src/include -D_GNU_SOURCE   -c -o gram.o
>> /home/peter/postgresql/src/backend/parser/gram.c
>> In file included from gram.y:12939:
>> scan.c:16246:23: warning: unused variable 'yyg' [-Wunused-variable]
>>    struct yyguts_t * yyg = (struct yyguts_t*)yyscanner; /* This var may be ...
>>                      ^
>> 1 warning generated.
>>
>> real  1m5.660s
>> user  1m5.344s
>> sys   0m0.083s
>> [peter at peter postgresql]$ time gcc -O2 -Wall -Wmissing-prototypes
>> -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels
>> -Wformat-security -fno-strict-aliasing -fwrapv -Wno-error -I. -I. -I
>> /home/peter/postgresql/src/include -D_GNU_SOURCE   -c -o gram.o
>> /home/peter/postgresql/src/backend/parser/gram.c
>> In file included from gram.y:12939:0:
>> scan.c: In function ‘yy_try_NUL_trans’:
>> scan.c:16246:23: warning: unused variable ‘yyg’ [-Wunused-variable]
>>
>> real  0m2.800s
>> user  0m2.688s
>> sys   0m0.094s
>>
>> That's obviously a difference way out of proportion to the total
>> difference in compile times.
>
> Maybe precompiled headers are not utilized when building with clang ? Clang does not automatically pick up the PCH from the first #include, you have to provide it at command-line.

Try reading that again... this is almost certainly not a frontend issue time.

-Eli