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From: "hubicka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/110377] New: Early VRP and IPA-PROP should work out
 value ranges from __builtin_unreacahble
Date: Fri, 23 Jun 2023 15:37:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110377

            Bug ID: 110377
           Summary: Early VRP and IPA-PROP should work out value ranges
                    from __builtin_unreacahble
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hubicka at gcc dot gnu.org
  Target Milestone: ---

In the following testcase
void test2(int);
void
test(int n)
{
        if (n > 5)
          __builtin_unreachable ();
        test2(n);
}
we should work out that value range of n passed to test2 is [INT_MIN,4].
This would help optimizing some code in libstdc++, which now uses similar
constructs to ensure known value ranges.

I think it is a common case where such unreachable test can be retrofited to
the SSA_NAME based on the fact that program can not terminate between
definition and the conditional.

We currently get:
  function  test/0 parameter descriptors:
    param #0 n used undescribed_use
  Jump functions of caller  test/0:
    callsite  test/0 -> test2/2 :=20
       param 0: PASS THROUGH: 0, op nop_expr
         value: 0x0, mask:
0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff=
fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
         Unknown VR
    callsite  test/0 -> __builtin_unreachable/1 :=