add load/store reverse instructions to the MD file with GCC

Question

I was working in an Embedded processor with GCC-4.6.4 version. I need to add load/store reverse instructions to the MD file. My instructions will look as below:

 LWX Rd,Ra,Rb
 operation: Addr := Ra + Rb
 Rd := *Addr (loading data with the opposite endianness)
 SWX Rd,Ra,Rb
 operation: Addr := Ra + Rb
 *Addr := Rd (storing data with the opposite endianness)

To add the above instructions in to md file I tried below pattern in md file

 (define_insn "movsi_rev"
 [(set (match_operand:SI 0 "nonimmediate_operand" "=d,m")
 (bswap: SI (match_operand:SI 1 "move_src_operand" "m,d")))]
 ""
 "@
 lwxt%0,%1,%0
 swxt%0,%1,%0"
 [(set_attr "type" "load,store")
 (set_attr "mode" "SI")
 (set_attr "length" "4,4")])

I wrote a small testcase which is generating swx instruction but the operands are more due to which it is failing in assembler phase

 ex: instead of swx r0,r2,r0 it is generating swx r0,r2,0,r0

can anyone please help me in removing the extra operand in the above instruction.

Answer 1

Are you trying to modify gcc itself for your particular target, or are just wanting to use these instructions in your own code? I can't think why the compiler would need to generate endian-reversed accesses itself
(I think it would be great if gcc supported a variable or type attribute specifying that the data should be accessed with reversed endianness, but that's not available at the moment).

Using the instruction directly from your code is pretty easy with inline assembler:

static inline uint32_t LMX(uint32_t * Ra, uint32_t Rb) {
 uint32_t Rd;
 asm(" lmx %[Rd], %[Ra], %[Rb]" :
 [Rd] "=r" (Rd) :
 [Ra] "r" (Ra), [Rb] "r" (Rb) );
 return Rd;
}

That should give you optimal code.