[sr #110729] m4 processing of large grammars slow

[Andres Freund]

URL:
  <https://savannah.gnu.org/support/?110729>

                 Summary: m4 processing of large grammars slow
                 Project: Bison
               Submitter: anarazel
               Submitted: Tue 20 Sep 2022 07:08:29 PM UTC
                Category: None
                Priority: 5 - Normal
                Severity: 3 - Normal
                  Status: None
                 Privacy: Public
             Assigned to: None
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
        Operating System: None


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Follow-up Comments:


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Date: Tue 20 Sep 2022 07:08:29 PM UTC By: Andres Freund <anarazel>
Hi,

translating postgres' bison grammars these days is the slowest build step of
building postgres. Interestingly enough the time isn't even spent building the
state machine or such, it's nearly all in m4.

Here's an example with postgres' main grammar. While there are other grammars
that are slower to build, they require prior build steps.

curl -o gram.y
'https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob_plain;f=src/backend/parser/gram.y;hb=HEAD'
$ /usr/bin/bison --trace=time -Wno-deprecated -o gram.c -d gram.y
bison (GNU Bison) 3.8.2
Execution times (seconds)
                          CPU user      CPU system    wall clock      
 reader                   0.044 ( 2%)   0.000 ( 0%)   0.044988 ( 2%)
 LR(0)                    0.038 ( 2%)   0.000 ( 0%)   0.038423 ( 2%)
 LALR(1)                  0.083 ( 4%)   0.003 (16%)   0.086182 ( 4%)
 parser action tables     0.149 ( 7%)   0.000 ( 0%)   0.149236 ( 7%)
 outputting parser        0.075 ( 3%)   0.003 (16%)   0.119665 ( 6%)
 running m4               1.518 (79%)   0.013 (66%)   1.454934 (76%)
 total time               1.914         0.020         1.900101

On slow systems this often ends up > 10s.

I don't know anything about m4, but perhaps there's something that can be done
to speed this step up?

Regards,

Andres







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