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From: "curiousdannii at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/100393] New: Very slow compilation of switch statement with
 thousands of cases
Date: Mon, 03 May 2021 11:46:51 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100393

            Bug ID: 100393
           Summary: Very slow compilation of switch statement with
                    thousands of cases
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: curiousdannii at gmail dot com
  Target Milestone: ---

I have a switch statement with almost 11,000 cases (produced by a decompile=
r),
which is taking almost 5 minutes to compile with GCC 10.2. Helpful people on
Stack Overflow suggested that it was a regression, that it only took 3 seco=
nds
to compile with GCC 8.4, and for comparison, only 1.3 seconds in Clang. I
haven't been able to confirm those results yet, but I'll look into doing so.

SO discussion: https://stackoverflow.com/q/67363813/2854284

Preprocessed source:
https://gist.githubusercontent.com/curiousdannii/9476375ff3ae22c403ce2a8132=
e6a5dc/raw/568f519f2f1b599e98c514f3609a4968d4153eed/functions_unsafe.i

The `-ftime-report -ftime-report-details` results (full results in the SO p=
age)
show that the slow part is in "tree switch lowering".

I also tried compiling it with `-fno-jump-tables`, which, rather than helpi=
ng,
made it much worse, taking over 33 minutes to compile.=