i386.md

gcc-you can use this code to learn something about gcc, and inquire further into linux,

{
  if (GET_CODE (operands[0]) == MEM && GET_CODE (operands[1]) == MEM)
    operands[0] = force_reg (HImode, operands[0]);
  ix86_compare_op0 = operands[0];
  ix86_compare_op1 = operands[1];
  DONE;
})

(define_expand "cmpqi"
  [(set (reg:CC 17)
	(compare:CC (match_operand:QI 0 "nonimmediate_operand" "")
		    (match_operand:QI 1 "general_operand" "")))]
  "TARGET_QIMODE_MATH"
{
  if (GET_CODE (operands[0]) == MEM && GET_CODE (operands[1]) == MEM)
    operands[0] = force_reg (QImode, operands[0]);
  ix86_compare_op0 = operands[0];
  ix86_compare_op1 = operands[1];
  DONE;
})

(define_insn "cmpdi_ccno_1_rex64"
  [(set (reg 17)
	(compare (match_operand:DI 0 "nonimmediate_operand" "r,?mr")
		 (match_operand:DI 1 "const0_operand" "n,n")))]
  "TARGET_64BIT && ix86_match_ccmode (insn, CCNOmode)"
  "@
   test{q}\t{%0, %0|%0, %0}
   cmp{q}\t{%1, %0|%0, %1}"
  [(set_attr "type" "test,icmp")
   (set_attr "length_immediate" "0,1")
   (set_attr "mode" "DI")])

(define_insn "*cmpdi_minus_1_rex64"
  [(set (reg 17)
	(compare (minus:DI (match_operand:DI 0 "nonimmediate_operand" "rm,r")
			   (match_operand:DI 1 "x86_64_general_operand" "re,mr"))
		 (const_int 0)))]
  "TARGET_64BIT && ix86_match_ccmode (insn, CCGOCmode)"
  "cmp{q}\t{%1, %0|%0, %1}"
  [(set_attr "type" "icmp")
   (set_attr "mode" "DI")])

(define_expand "cmpdi_1_rex64"
  [(set (reg:CC 17)
	(compare:CC (match_operand:DI 0 "nonimmediate_operand" "")
		    (match_operand:DI 1 "general_operand" "")))]
  "TARGET_64BIT"
  "")

(define_insn "cmpdi_1_insn_rex64"
  [(set (reg 17)
	(compare (match_operand:DI 0 "nonimmediate_operand" "mr,r")
		 (match_operand:DI 1 "x86_64_general_operand" "re,mr")))]
  "TARGET_64BIT && ix86_match_ccmode (insn, CCmode)"
  "cmp{q}\t{%1, %0|%0, %1}"
  [(set_attr "type" "icmp")
   (set_attr "mode" "DI")])


(define_insn "*cmpsi_ccno_1"
  [(set (reg 17)
	(compare (match_operand:SI 0 "nonimmediate_operand" "r,?mr")
		 (match_operand:SI 1 "const0_operand" "n,n")))]
  "ix86_match_ccmode (insn, CCNOmode)"
  "@
   test{l}\t{%0, %0|%0, %0}
   cmp{l}\t{%1, %0|%0, %1}"
  [(set_attr "type" "test,icmp")
   (set_attr "length_immediate" "0,1")
   (set_attr "mode" "SI")])

(define_insn "*cmpsi_minus_1"
  [(set (reg 17)
	(compare (minus:SI (match_operand:SI 0 "nonimmediate_operand" "rm,r")
			   (match_operand:SI 1 "general_operand" "ri,mr"))
		 (const_int 0)))]
  "ix86_match_ccmode (insn, CCGOCmode)"
  "cmp{l}\t{%1, %0|%0, %1}"
  [(set_attr "type" "icmp")
   (set_attr "mode" "SI")])

(define_expand "cmpsi_1"
  [(set (reg:CC 17)
	(compare:CC (match_operand:SI 0 "nonimmediate_operand" "rm,r")
		    (match_operand:SI 1 "general_operand" "ri,mr")))]
  ""
  "")

(define_insn "*cmpsi_1_insn"
  [(set (reg 17)
	(compare (match_operand:SI 0 "nonimmediate_operand" "rm,r")
		 (match_operand:SI 1 "general_operand" "ri,mr")))]
  "(GET_CODE (operands[0]) != MEM || GET_CODE (operands[1]) != MEM)
    && ix86_match_ccmode (insn, CCmode)"
  "cmp{l}\t{%1, %0|%0, %1}"
  [(set_attr "type" "icmp")
   (set_attr "mode" "SI")])

(define_insn "*cmphi_ccno_1"
  [(set (reg 17)
	(compare (match_operand:HI 0 "nonimmediate_operand" "r,?mr")
		 (match_operand:HI 1 "const0_operand" "n,n")))]
  "ix86_match_ccmode (insn, CCNOmode)"
  "@
   test{w}\t{%0, %0|%0, %0}
   cmp{w}\t{%1, %0|%0, %1}"
  [(set_attr "type" "test,icmp")
   (set_attr "length_immediate" "0,1")
   (set_attr "mode" "HI")])

(define_insn "*cmphi_minus_1"
  [(set (reg 17)
	(compare (minus:HI (match_operand:HI 0 "nonimmediate_operand" "rm,r")
			   (match_operand:HI 1 "general_operand" "ri,mr"))
		 (const_int 0)))]
  "ix86_match_ccmode (insn, CCGOCmode)"
  "cmp{w}\t{%1, %0|%0, %1}"
  [(set_attr "type" "icmp")
   (set_attr "mode" "HI")])

(define_insn "*cmphi_1"
  [(set (reg 17)
	(compare (match_operand:HI 0 "nonimmediate_operand" "rm,r")
		 (match_operand:HI 1 "general_operand" "ri,mr")))]
  "(GET_CODE (operands[0]) != MEM || GET_CODE (operands[1]) != MEM)
   && ix86_match_ccmode (insn, CCmode)"
  "cmp{w}\t{%1, %0|%0, %1}"
  [(set_attr "type" "icmp")
   (set_attr "mode" "HI")])

(define_insn "*cmpqi_ccno_1"
  [(set (reg 17)
	(compare (match_operand:QI 0 "nonimmediate_operand" "q,?mq")
		 (match_operand:QI 1 "const0_operand" "n,n")))]
  "ix86_match_ccmode (insn, CCNOmode)"
  "@
   test{b}\t{%0, %0|%0, %0}
   cmp{b}\t{$0, %0|%0, 0}"
  [(set_attr "type" "test,icmp")
   (set_attr "length_immediate" "0,1")
   (set_attr "mode" "QI")])

(define_insn "*cmpqi_1"
  [(set (reg 17)
	(compare (match_operand:QI 0 "nonimmediate_operand" "qm,q")
		 (match_operand:QI 1 "general_operand" "qi,mq")))]
  "(GET_CODE (operands[0]) != MEM || GET_CODE (operands[1]) != MEM)
    && ix86_match_ccmode (insn, CCmode)"
  "cmp{b}\t{%1, %0|%0, %1}"
  [(set_attr "type" "icmp")
   (set_attr "mode" "QI")])

(define_insn "*cmpqi_minus_1"
  [(set (reg 17)
	(compare (minus:QI (match_operand:QI 0 "nonimmediate_operand" "qm,q")
			   (match_operand:QI 1 "general_operand" "qi,mq"))
		 (const_int 0)))]
  "ix86_match_ccmode (insn, CCGOCmode)"
  "cmp{b}\t{%1, %0|%0, %1}"
  [(set_attr "type" "icmp")
   (set_attr "mode" "QI")])

(define_insn "*cmpqi_ext_1"
  [(set (reg 17)
	(compare
	  (match_operand:QI 0 "general_operand" "Qm")
	  (subreg:QI
	    (zero_extract:SI
	      (match_operand 1 "ext_register_operand" "Q")
	      (const_int 8)
	      (const_int 8)) 0)))]
  "!TARGET_64BIT && ix86_match_ccmode (insn, CCmode)"
  "cmp{b}\t{%h1, %0|%0, %h1}"
  [(set_attr "type" "icmp")
   (set_attr "mode" "QI")])

(define_insn "*cmpqi_ext_1_rex64"
  [(set (reg 17)
	(compare
	  (match_operand:QI 0 "register_operand" "Q")
	  (subreg:QI
	    (zero_extract:SI
	      (match_operand 1 "ext_register_operand" "Q")
	      (const_int 8)
	      (const_int 8)) 0)))]
  "TARGET_64BIT && ix86_match_ccmode (insn, CCmode)"
  "cmp{b}\t{%h1, %0|%0, %h1}"
  [(set_attr "type" "icmp")
   (set_attr "mode" "QI")])

(define_insn "*cmpqi_ext_2"
  [(set (reg 17)
	(compare
	  (subreg:QI
	    (zero_extract:SI
	      (match_operand 0 "ext_register_operand" "Q")
	      (const_int 8)
	      (const_int 8)) 0)
	  (match_operand:QI 1 "const0_operand" "n")))]
  "ix86_match_ccmode (insn, CCNOmode)"
  "test{b}\t%h0, %h0"
  [(set_attr "type" "test")
   (set_attr "length_immediate" "0")
   (set_attr "mode" "QI")])

(define_expand "cmpqi_ext_3"
  [(set (reg:CC 17)
	(compare:CC
	  (subreg:QI
	    (zero_extract:SI
	      (match_operand 0 "ext_register_operand" "")
	      (const_int 8)
	      (const_int 8)) 0)
	  (match_operand:QI 1 "general_operand" "")))]
  ""
  "")

(define_insn "cmpqi_ext_3_insn"
  [(set (reg 17)
	(compare
	  (subreg:QI
	    (zero_extract:SI
	      (match_operand 0 "ext_register_operand" "Q")
	      (const_int 8)
	      (const_int 8)) 0)
	  (match_operand:QI 1 "general_operand" "Qmn")))]
  "!TARGET_64BIT && ix86_match_ccmode (insn, CCmode)"
  "cmp{b}\t{%1, %h0|%h0, %1}"
  [(set_attr "type" "icmp")
   (set_attr "mode" "QI")])

(define_insn "cmpqi_ext_3_insn_rex64"
  [(set (reg 17)
	(compare
	  (subreg:QI
	    (zero_extract:SI
	      (match_operand 0 "ext_register_operand" "Q")
	      (const_int 8)
	      (const_int 8)) 0)
	  (match_operand:QI 1 "nonmemory_operand" "Qn")))]
  "TARGET_64BIT && ix86_match_ccmode (insn, CCmode)"
  "cmp{b}\t{%1, %h0|%h0, %1}"
  [(set_attr "type" "icmp")
   (set_attr "mode" "QI")])

(define_insn "*cmpqi_ext_4"
  [(set (reg 17)
	(compare
	  (subreg:QI
	    (zero_extract:SI
	      (match_operand 0 "ext_register_operand" "Q")
	      (const_int 8)
	      (const_int 8)) 0)
	  (subreg:QI
	    (zero_extract:SI
	      (match_operand 1 "ext_register_operand" "Q")
	      (const_int 8)
	      (const_int 8)) 0)))]
  "ix86_match_ccmode (insn, CCmode)"
  "cmp{b}\t{%h1, %h0|%h0, %h1}"
  [(set_attr "type" "icmp")
   (set_attr "mode" "QI")])

;; These implement float point compares.
;; %%% See if we can get away with VOIDmode operands on the actual insns,
;; which would allow mix and match FP modes on the compares.  Which is what
;; the old patterns did, but with many more of them.

(define_expand "cmpxf"
  [(set (reg:CC 17)
	(compare:CC (match_operand:XF 0 "cmp_fp_expander_operand" "")
		    (match_operand:XF 1 "cmp_fp_expander_operand" "")))]
  "!TARGET_64BIT && TARGET_80387"
{
  ix86_compare_op0 = operands[0];
  ix86_compare_op1 = operands[1];
  DONE;
})

(define_expand "cmptf"
  [(set (reg:CC 17)
	(compare:CC (match_operand:TF 0 "cmp_fp_expander_operand" "")
		    (match_operand:TF 1 "cmp_fp_expander_operand" "")))]
  "TARGET_80387"
{
  ix86_compare_op0 = operands[0];
  ix86_compare_op1 = operands[1];
  DONE;
})

(define_expand "cmpdf"
  [(set (reg:CC 17)
	(compare:CC (match_operand:DF 0 "cmp_fp_expander_operand" "")
		    (match_operand:DF 1 "cmp_fp_expander_operand" "")))]
  "TARGET_80387 || TARGET_SSE2"
{
  ix86_compare_op0 = operands[0];
  ix86_compare_op1 = operands[1];
  DONE;
})

(define_expand "cmpsf"
  [(set (reg:CC 17)
	(compare:CC (match_operand:SF 0 "cmp_fp_expander_operand" "")
		    (match_operand:SF 1 "cmp_fp_expander_operand" "")))]
  "TARGET_80387 || TARGET_SSE"
{
  ix86_compare_op0 = operands[0];
  ix86_compare_op1 = operands[1];
  DONE;
})

;; FP compares, step 1:
;; Set the FP condition codes.
;;
;; CCFPmode	compare with exceptions
;; CCFPUmode	compare with no exceptions

;; %%% It is an unfortunate fact that ftst has no non-popping variant,
;; and that fp moves clobber the condition codes, and that there is
;; currently no way to describe this fact to reg-stack.  So there are
;; no splitters yet for this.

;; %%% YIKES!  This scheme does not retain a strong connection between 
;; the real compare and the ultimate cc0 user, so CC_REVERSE does not
;; work!  Only allow tos/mem with tos in op 0.
;;
;; Hmm, of course, this is what the actual _hardware_ does.  Perhaps
;; things aren't as bad as they sound...

(define_insn "*cmpfp_0"
  [(set (match_operand:HI 0 "register_operand" "=a")
	(unspec:HI
	  [(compare:CCFP (match_operand 1 "register_operand" "f")
		         (match_operand 2 "const0_operand" "X"))]
	  UNSPEC_FNSTSW))]
  "TARGET_80387
   && FLOAT_MODE_P (GET_MODE (operands[1]))
   && GET_MODE (operands[1]) == GET_MODE (operands[2])"
{
  if (find_regno_note (insn, REG_DEAD, REGNO (operands[1])))
    return "ftst\;fnstsw\t%0\;fstp\t%y0";
  else
    return "ftst\;fnstsw\t%0";
}
  [(set_attr "type" "multi")
   (set_attr "mode" "unknownfp")])

;; We may not use "#" to split and emit these, since the REG_DEAD notes
;; used to manage the reg stack popping would not be preserved.

(define_insn "*cmpfp_2_sf"
  [(set (reg:CCFP 18)
	(compare:CCFP
	  (match_operand:SF 0 "register_operand" "f")
	  (match_operand:SF 1 "nonimmediate_operand" "fm")))]
  "TARGET_80387"
  "* return output_fp_compare (insn, operands, 0, 0);"
  [(set_attr "type" "fcmp")
   (set_attr "mode" "SF")])

(define_insn "*cmpfp_2_sf_1"
  [(set (match_operand:HI 0 "register_operand" "=a")
	(unspec:HI
	  [(compare:CCFP
	     (match_operand:SF 1 "register_operand" "f")
	     (match_operand:SF 2 "nonimmediate_operand" "fm"))]
	  UNSPEC_FNSTSW))]
  "TARGET_80387"
  "* return output_fp_compare (insn, operands, 2, 0);"
  [(set_attr "type" "fcmp")
   (set_attr "mode" "SF")])

(define_insn "*cmpfp_2_df"
  [(set (reg:CCFP 18)
	(compare:CCFP
	  (match_operand:DF 0 "register_operand" "f")
	  (match_operand:DF 1 "nonimmediate_operand" "fm")))]
  "TARGET_80387"
  "* return output_fp_compare (insn, operands, 0, 0);"
  [(set_attr "type" "fcmp")
   (set_attr "mode" "DF")])

(define_insn "*cmpfp_2_df_1"
  [(set (match_operand:HI 0 "register_operand" "=a")
	(unspec:HI
	  [(compare:CCFP
	     (match_operand:DF 1 "register_operand" "f")
	     (match_operand:DF 2 "nonimmediate_operand" "fm"))]
	  UNSPEC_FNSTSW))]
  "TARGET_80387"
  "* return output_fp_compare (insn, operands, 2, 0);"
  [(set_attr "type" "multi")
   (set_attr "mode" "DF")])

(define_insn "*cmpfp_2_xf"
  [(set (reg:CCFP 18)
	(compare:CCFP
	  (match_operand:XF 0 "register_operand" "f")
	  (match_operand:XF 1 "register_operand" "f")))]
  "!TARGET_64BIT && TARGET_80387"
  "* return output_fp_compare (insn, operands, 0, 0);"
  [(set_attr "type" "fcmp")
   (set_attr "mode" "XF")])

(define_insn "*cmpfp_2_tf"
  [(set (reg:CCFP 18)
	(compare:CCFP
	  (match_operand:TF 0 "register_operand" "f")
	  (match_operand:TF 1 "register_operand" "f")))]
  "TARGET_80387"