https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64681
David <gccbugzilla at limegreensocks dot com> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |gccbugzilla@limegreensocks. | |com --- Comment #4 from David <gccbugzilla at limegreensocks dot com> --- Until very recently, *none* of the modifiers were documented. In the current docs (https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#i386Operandmodifiers), only the i386 modifiers are doc'ed. That said, you can always check out the gcc source. For example: The comments above output_asm_insn in https://gcc.gnu.org/viewcvs/gcc/trunk/gcc/final.c?view=co&content-type=text%2Fplain describe some global modifiers. For i386-specific modifiers, look above ix86_print_operand in https://gcc.gnu.org/viewcvs/gcc/trunk/gcc/config/i386/i386.c?view=co&content-type=text%2Fplain For arm, look above arm_print_operand in https://gcc.gnu.org/viewcvs/gcc/trunk/gcc/config/arm/arm.c?view=co&content-type=text%2Fplain Presumably there are similar sections for other configs.