On Tue, Nov 14, 2023 at 6:51 PM Jakub Jelinek <ja...@redhat.com> wrote:
>
> On Mon, Nov 13, 2023 at 10:49:23PM +0100, Uros Bizjak wrote:
> > Combine wants to combine following instructions into an insn that can
> > perform both an (arithmetic) operation and set the condition code. During
> > the conversion a new RTX is created, and combine passes the RTX code of the
> > innermost RTX expression of the CC use insn in which CC reg is used to
> > SELECT_CC_MODE, to determine the new mode of the comparison:
> >
> > Trying 5 -> 8:
> > 5: r98:DI=0xd7
> > 8: flags:CCZ=cmp(r98:DI,0)
> > REG_EQUAL cmp(0xd7,0)
> > Failed to match this instruction:
> > (parallel [
> > (set (reg:CC 17 flags)
> > (compare:CC (const_int 215 [0xd7])
> > (const_int 0 [0])))
> > (set (reg/v:DI 98 [ flags ])
> > (const_int 215 [0xd7]))
> > ])
> >
> > where:
> >
> > (insn 5 2 6 2 (set (reg/v:DI 98 [ flags ])
> > (const_int 215 [0xd7])) "pr112494.c":8:8 84 {*movdi_internal}
> > (nil))
> >
> > (insn 8 7 11 2 (set (reg:CCZ 17 flags)
> > (compare:CCZ (reg/v:DI 98 [ flags ])
> > (const_int 0 [0]))) "pr112494.c":11:9 8 {*cmpdi_ccno_1}
> > (expr_list:REG_EQUAL (compare:CCZ (const_int 215 [0xd7])
> > (const_int 0 [0]))
> > (nil)))
> >
> > x86_cc_mode (AKA SELECT_CC_MODE) is not prepared to handle random RTX
> > codes and triggers gcc_unreachable() when SET RTX code is passed to it.
> > The patch removes gcc_unreachable() and returns CCmode for unknown
> > RTX codes, so combine can try various combinations involving CC reg
> > without triggering ICE.
> >
> > Please note that x86 MOV instructions do not set flags, so the above
> > combination is not recognized as a valid x86 instruction.
> >
> > PR target/112494
> >
> > gcc/ChangeLog:
> >
> > * config/i386/i386.cc (ix86_cc_mode) [default]: Return CCmode.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * gcc.target/i386/pr112494.c: New test.
> >
> > Patch was bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.
>
> For me the test ICEs with RTL checking on both x86_64-linux and i686-linux.
> pr112494.c:17:1: internal compiler error: RTL check: expected elt 0 type 'e'
> or 'u', have 'E' (rtx unspec) in try_combine, at combine.cc:3237
> This is on
> 3236 /* Just replace the CC reg with a new mode. */
> 3237 SUBST (XEXP (*cc_use_loc, 0), newpat_dest);
> 3238 undobuf.other_insn = cc_use_insn;
> in combine.cc, where *cc_use_loc is
> (unspec:DI [
> (reg:CC 17 flags)
> ] UNSPEC_PUSHFL)
> on which XEXP (guess combine assumes CC must be used inside of a
> comparison).
Can you please fill out a bugreport, so the bug can be properly
tiaged? I don't think there is anything wrong with new patterns, at
least the documentation doesn't say that CC_reg can't be used in
unspecs and other non-compare RTXes.
Thanks,
Uros.
