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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/98292] New: Optimize away C return; in
 function returning integral/pointer
Date: Tue, 15 Dec 2020 16:29:36 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98292

            Bug ID: 98292
           Summary: Optimize away C return; in function returning
                    integral/pointer
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
  Target Milestone: ---

This is related to the PR94779, but that one talks just about switches.

int
foo (int x)
{
  if (x > 20)
    return x;
}

int
bar (int x)
{
  if (x > 30)
    return 30;
}

int
baz (int x)
{
  if (x > 40)
    return x + 1U;
}

int
qux (int x)
{
  if (x > 50)
    return x | 32;
}

is optimized to just the return statement in C++ (because we add
__builtin_unreachable() in that case), but not in C.
I think we should do that also in C, provided that there are no non-debug
non-CLOBBER stmts on the branch with GIMPLE_RETURN without argument, and
provided that on the other branch there are just very few cheap stmts
guaranteed not to invoke UB/trap (appart from debug/CLOBBER stmts) plus the
GIMPLE_RETURN with argument.
LLVM seems to optimize that (though not sure if it isn't because it incorre=
ctly
adds something like __builtin_unreachable () even for C).=