[Bug rtl-optimization/97497] New: gcse wrong code generation with partial register clobbers

[krebbel at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97497

            Bug ID: 97497
           Summary: gcse wrong code generation with partial register
                    clobbers
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: krebbel at gcc dot gnu.org
  Target Milestone: ---

Compiling the attached testcase produces wrong code on IBM Z:

cc1 t.c -m31 -mzarch -march=z900 -O2 -fpic -o t.s

foo:
        stm     %r11,%r15,44(%r15)
        larl    %r12,_GLOBAL_OFFSET_TABLE_
        lr      %r11,%r2
        l       %r1,t@GOT(%r12)
        ahi     %r15,-96
        lhi     %r2,1
        l       %r3,0(%r1)
        brasl   %r14,bar@PLT
        ltr     %r11,%r11
        jne     .L8
        lhi     %r2,1
        l       %r3,0                 <--- dereference address 0
        brasl   %r14,bar@PLT
        l       %r4,152(%r15)
        lm      %r11,%r15,140(%r15)
        br      %r4
.L8:
        lhi     %r3,1
        l       %r2,0                 <--- dereference address 0
        brasl   %r14,baz@PLT
        lhi     %r2,1
        l       %r3,0
        brasl   %r14,bar@PLT
        l       %r4,152(%r15)
        lm      %r11,%r15,140(%r15)
        br      %r4


gcse decides to remove the load from t in the subsequent bbs but does not
generate the load into a temp reg in the first bb leaving the bbs loading from
an uninitialized pseudo.

With -mzarch -m31 we have the GOT pointer marked as partially clobbered. The
loads from t use the GOT pointer explicitly in the RTX. Since this patch r12 is
considered to be fully clobbered by call insns:

commit a4dfaad2e5594d871fe00a1116005e28f95d644e (refs/bisect/bad)
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Mon Sep 30 16:20:44 2019 +0000

    Remove global call sets: gcse.c

    This is another case in which we can conservatively treat partial
    kills as full kills.  Again this is in principle a bug fix for
    TARGET_HARD_REGNO_CALL_PART_CLOBBERED targets, but in practice
    it probably doesn't make a difference.

    2019-09-30  Richard Sandiford  <richard.sandiford@arm.com>

    gcc/
            * gcse.c: Include function-abi.h.
            (compute_hash_table_work): Use insn_callee_abi to get the ABI of
            the call insn target.  Invalidate partially call-clobbered
            registers as well as fully call-clobbered ones.

    From-SVN: r276323

Now the RTX for t which references r12 is considered to be not available
anymore in the later bbs due to r12 being clobbered by the calls. Hence no load
of the original expression is being emitted.