[Bug debug/101266] ICE with -g: in loc_list_from_tree_1, at dwarf2out.c:19421

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101266

--- Comment #11 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Roger Sayle <sayle@gcc.gnu.org>:

https://gcc.gnu.org/g:e46d96d20bbfa786d30c9371631da00939845f05

commit r17-2413-ge46d96d20bbfa786d30c9371631da00939845f05
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Wed Jul 15 12:26:16 2026 +0100

    PR middle-end/123236: Simplify (int)((long long)x >> 4)

    This patch addresses a code quality regression on x86_64 related to
    PR 123236.  That original PR (and the related PR 101266) concern tree
    level optimizations, where this problem should also be fixed, but it
    also reveals a regression in the RTL optimizers.

    A motivating test case (on x86_64) is:

    int bar(int a) {
      long long t = a;
      return t >> 4;
    }

    Currently -O2 generates a 64-bit shift:
    bar:    movslq  %edi, %rax
            sarq    $4, %rax
            ret

    with this patch we now generate a 32-bit shift:
    bar:    movl    %edi, %eax
            sarl    $4, %eax
            ret

    The underlying cause of the RTL-level regression is that some
    RTL expressions that were previously expressed as {ZERO,SIGN}_EXTEND
    are now sometimes represented as the equivalent {ZERO,SIGN}_EXTRACT,
    and that not all simplifications of TRUNCATE({ZERO,SIGN}_EXTEND) are
    implemented for TRUNCATE({ZERO,SIGN}_EXTRACT).

    Thanks to Segher, simplify_rtx does handle some truncations of extracts,
    see https://gcc.gnu.org/pipermail/gcc-patches/2016-November/463629.html
    but unfortunately this code (and subsequent tweaks) doesn't quite match
    the cases that appear here.

    2026-07-15  Roger Sayle  <roger@nextmovesoftware.com>

    gcc/ChangeLog
            PR rtl-optimization/123236
            * simplify-rtx.cc (simplify_context::simplify_truncation): Handle
            cases where a ZERO_EXTRACT or SIGN_EXTRACT has a different mode
            to (but at least as wide as) its first operand.

    gcc/testsuite/ChangeLog
            PR rtl-optimization/123236
            * gcc.target/i386/pr123236-1.c: New test case.