https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118623
--- Comment #12 from Hongtao Liu <liuhongt at gcc dot gnu.org> ---
1370Trying 35 -> 20:
1371 35: flags:CCC=cmp(zero_extract(r104:SI,0x1,r105:SI#0),0)
1372 REG_DEAD r104:SI
1373 REG_DEAD r105:SI
1374 20: pc={(flags:CCC!=0)?L26:pc}
1375 REG_BR_PROB 1073741831
1376 REG_DEAD flags:CCZ
1377Successfully matched this instruction:
1378(set (pc)
1379 (if_then_else (ne (zero_extract:SI (reg/v:SI 104 [ g ])
1380 (const_int 1 [0x1])
1381 (subreg:QI (reg:SI 105 [ _1 ]) 0))
1382 (const_int 0 [0]))
1383 (label_ref:DI 26)
1384 (pc)))
The problem is here, {(flags:CCC!=0)?L26:pc} means when bit CCC is not set
which should be equal to
(eq (zero_extract:SI (reg/v:SI 104 [ g ])
1380 (const_int 1 [0x1])
1381 (subreg:QI (reg:SI 105 [ _1 ]) 0) 0).
Not *ne*, Looks like rtl simplication handle CFLAGS differently from backend?
In i386.md, there's explicitly reverse for the condition code when handling
zero_extract against zero with CCCmode.
19178(define_insn_and_split "*jcc_bt<mode>"
19179 [(set (pc)
19180 (if_then_else (match_operator 0 "bt_comparison_operator"
19181 [(zero_extract:SWI48
19182 (match_operand:SWI48 1 "nonimmediate_operand")
19183 (const_int 1)
19184 (match_operand:QI 2 "nonmemory_operand"))
19185 (const_int 0)])
19186 (label_ref (match_operand 3))
19187 (pc)))
19188 (clobber (reg:CC FLAGS_REG))]
19189 "(TARGET_USE_BT || optimize_function_for_size_p (cfun))
19190 && (CONST_INT_P (operands[2])
19191 ? (INTVAL (operands[2]) < GET_MODE_BITSIZE (<MODE>mode)
19192 && INTVAL (operands[2])
19193 >= (optimize_function_for_size_p (cfun) ? 8 : 32))
19194 : !memory_operand (operands[1], <MODE>mode))
19195 && ix86_pre_reload_split ()"
19196 "#"
19197 "&& 1"
19198 [(set (reg:CCC FLAGS_REG)
19199 (compare:CCC
19200 (zero_extract:SWI48
19201 (match_dup 1)
19202 (const_int 1)
19203 (match_dup 2))
19204 (const_int 0)))
19205 (set (pc)
19206 (if_then_else (match_op_dup 0 [(reg:CCC FLAGS_REG) (const_int 0)])
19207 (label_ref (match_dup 3))
19208 (pc)))]
19209{
19210 operands[0] = shallow_copy_rtx (operands[0]);
19211 PUT_CODE (operands[0], reverse_condition (GET_CODE (operands[0])));
19212})
