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From: "cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/111778] PowerPC constant code change uses an undefined
 shift
Date: Thu, 12 Oct 2023 20:19:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111778

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Michael Meissner <meissner@gcc.gnu.or=
g>:

https://gcc.gnu.org/g:611eef7609f732db65c119a7eab6d50a5fdd5985

commit r14-4600-g611eef7609f732db65c119a7eab6d50a5fdd5985
Author: Michael Meissner <meissner@linux.ibm.com>
Date:   Thu Oct 12 16:17:59 2023 -0400

    PR111778, PowerPC: Do not depend on an undefined shift

    I was building a cross compiler to PowerPC on my x86_86 workstation with
the
    latest version of GCC on October 11th.  I could not build the compiler =
on
the
    x86_64 system as it died in building libgcc.  I looked into it, and I
    discovered the compiler was recursing until it ran out of stack space. =
 If
I
    build a native compiler with the same sources on a PowerPC system, it
builds
    fine.

    I traced this down to a change made around October 10th:

    | commit 8f1a70a4fbcc6441c70da60d4ef6db1e5635e18a (HEAD)
    | Author: Jiufu Guo <guojiufu@linux.ibm.com>
    | Date:   Tue Jan 10 20:52:33 2023 +0800
    |
    |   rs6000: build constant via li/lis;rldicl/rldicr
    |
    |   If a constant is possible left/right cleaned on a rotated value from
    |   a negative value of "li/lis".  Then, using "li/lis ; rldicl/rldicr"
    |   to build the constant.

    The code was doing a -1 << 64 which is undefined behavior because diffe=
rent
    machines produce different results.  On the x86_64 system, (-1 << 64)
produces
    -1 while on a PowerPC 64-bit system, (-1 << 64) produces 0.  The x86_64
then
    recurses until the stack runs out of space.

    If I apply this patch, the compiler builds fine on both x86_64 as a Pow=
erPC
    crosss compiler and on a native PowerPC system.

    2023-10-12  Michael Meissner  <meissner@linux.ibm.com>

    gcc/

            PR target/111778
            * config/rs6000/rs6000.cc (can_be_built_by_li_lis_and_rldicl):
Protect
            code from shifts that are undefined.
            (can_be_built_by_li_lis_and_rldicr): Likewise.
            (can_be_built_by_li_and_rldic): Protect code from shifts that
            undefined.  Also replace uses of 1ULL with HOST_WIDE_INT_1U.=