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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/106809] [12/13 regression] large bison
 grammars compilation got a lot slower, mainly due to -Wuninitialized
Date: Fri, 02 Sep 2022 08:31:00 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106809

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
A testcase for this corner case is the following.  At -O0 it is
-Wuninitialized, at -O1 it is FRE:

 tree FRE                           :   5.55 ( 70%)   0.00 (  0%)   5.56 ( =
67%)
   16k (  0%)

and at -O2 PRE will finally make use of the value numbers, hoisting the
load and optimizing the function to a single conditional.

 tree PRE                           :   2.80 ( 27%)   0.02 (  6%)   2.82 ( =
26%)
   17k (  0%)
 tree FRE                           :   5.56 ( 54%)   0.01 (  3%)   5.57 ( =
52%)
   16k (  0%)

The -On behavior would be a regression from GCC 9 where the new VN was
introduced.

int foo (int x, int *val)
{
  switch (x)
    {
#define C(n) \
    case n + 0: return *val; \
    case n + 1: return *val; \
    case n + 2: return *val; \
    case n + 3: return *val; \
    case n + 4: return *val; \
    case n + 5: return *val; \
    case n + 6: return *val; \
    case n + 7: return *val; \
    case n + 8: return *val; \
    case n + 9: return *val;
#define C1(n) \
    C(n+00) C(n+10) C(n+20) C(n+30) C(n+40) \
    C(n+50) C(n+60) C(n+70) C(n+80) C(n+90)
#define C10(n) \
    C1(n+000) C1(n+100) C1(n+200) C1(n+300) C1(n+400) \
    C1(n+500) C1(n+600) C1(n+700) C1(n+800) C1(n+900)
    C10(1000)
    }
  return 0;
}=