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).
Jakub
