From the file gcc/config/i386/i386.c of the GCC sources:
b -- print the QImode name of the register for the indicated operand. %b0 would print %al if operands[0] is reg 0. w -- likewise, print the HImode name of the register. k -- likewise, print the SImode name of the register. q -- likewise, print the DImode name of the register. x -- likewise, print the V4SFmode name of the register. t -- likewise, print the V8SFmode name of the register. g -- likewise, print the V16SFmode name of the register. h -- print the QImode name for a "high" register, either ah, bh, ch or dh.
Similarly from gcc/config/i386/contraints.md:
;; We use the Y prefix to denote any number of conditional register sets: ;; z First SSE register. ;; i SSE2 inter-unit moves to SSE register enabled ;; j SSE2 inter-unit moves from SSE register enabled ;; m MMX inter-unit moves to MMX register enabled ;; n MMX inter-unit moves from MMX register enabled ;; a Integer register when zero extensions with AND are disabled ;; p Integer register when TARGET_PARTIAL_REG_STALL is disabled ;; f x87 register when 80387 floating point arithmetic is enabled ;; r SSE regs not requiring REX prefix when prefixes avoidance is enabled ;; and all SSE regs otherwise
This file also defines a “Yk” constraint but I don’t know if how well it would work in an asm statement:
(define_register_constraint "Yk" "TARGET_AVX512F ? MASK_EVEX_REGS : NO_REGS" "@internal Any mask register that can be used as predicate, i.e. k1-k7.")
Note this is all copied from the latest SVN revision. I don’t know what release of GCC, if any, the particular modifiers and constraints you’re interested in were added.