Despite them being ignored by the CPU, gas issues segment override prefixes for other than FS/GS in 64-bit mode. If doing so at all, it should clearly do this correctly. Determining the default segment, however, requires to take into consideration RegRex (so far, RSP, RBP, R12, and R13 were all treated equally here). 2012-07-24 Jan Beulich * config/tc-i386-intel.c (build_modrm_byte): Split determining default segment from figuring out encoding. Honor RegRex for the former. --- a/gas/config/tc-i386.c +++ b/gas/config/tc-i386.c @@ -5733,18 +5733,14 @@ build_modrm_byte (void) i.sib.base = i.base_reg->reg_num; /* x86-64 ignores REX prefix bit here to avoid decoder complications. */ - if ((i.base_reg->reg_num & 7) == EBP_REG_NUM) - { + if (!(i.base_reg->reg_flags & RegRex) + && (i.base_reg->reg_num == EBP_REG_NUM + || i.base_reg->reg_num == ESP_REG_NUM)) default_seg = &ss; - if (i.disp_operands == 0) - { - fake_zero_displacement = 1; - i.types[op].bitfield.disp8 = 1; - } - } - else if (i.base_reg->reg_num == ESP_REG_NUM) + if (i.base_reg->reg_num == 5 && i.disp_operands == 0) { - default_seg = &ss; + fake_zero_displacement = 1; + i.types[op].bitfield.disp8 = 1; } i.sib.scale = i.log2_scale_factor; if (i.index_reg == 0)