This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Unable to match instruction pattern

Thanks Richard,

That worked just as you suggested, and ... when I removed my zero_extendhisi2 the IIRC the target-independent optabs code generated exactly the same sequence.

Interestingly, I noticed that changing from my define_insn "zero_extendhisi2" back to the define_expand "zero_extendhisi2" caused some optimisation opportunities to be missed where an expression calculated in 32 bits could equally well performed at the natural 16 bit word width (because the result of the calculation was truncated to 16 bits) :-)

Cheers, Paul.

On 12/04/14 04:21, Richard Sandiford wrote: writes:
Found it ...

I had

(define_expand "zero_extendhisi2"
          (set (subreg:HI (match_operand:SI 0 "general_operand" "")0)
                          (match_operand:HI 1 "general_operand" ""))
          (set (subreg:HI (match_dup 0)2)
                          (const_int 0))
FWIW, in general you shouldn't use subregs in expanders, but instead use
something like:

(define_expand "zero_extendhisi2"
   [(set (match_operand:SI 0 "general_operand" "")
         (zero_extend:SI (match_operand:HI 1 "general_operand" "")))]
   emit_move_insn (gen_lowpart (HImode, operands[0]), operands[1]);
   emit_move_insn (gen_highpart (HImode, operands[0]), const0_rtx);

This allows MEMs to be simplified to smaller MEMs, which is preferable
to keeping them as SUBREGs.  It also allows subregs of hard registers
to be simplified to simple REGs and handles constants correctly.

IMO SUBREGs of MEMs should go away.  There's this strange convention
that (subreg (mem ...)) is treated as a register_operand before reload,
which is why:

              if( !register_operand( operands[0], <MODE>mode )
                  && !register_operand( operands[1], <MODE>mode ))
                /* one of the operands must be in a register.  */
                  operands[1] = copy_to_mode_reg (<MODE>mode, operands[1]);

wouldn't trigger for a move between a constant and a subreg of a MEM.

OTOH, you should be better off not defining the zero_extendhisi2 at all,
since IIRC the target-independent optabs code would generate the same
sequence for you.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]