https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126433
--- Comment #2 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Uroš Bizjak from comment #1)
> Created attachment 65143 [details]
> Patch in testing.
Hm, this won't fly... it hits some GCC oddity:
(define_insn "*bswaphi2"
[(set (match_operand:HI 0 "register_operand" "=Q,r")
(bswap:HI (match_operand:HI 1 "register_operand" "0,0")))
...
"@
xchg{b}\t{%h0, %b0|%b0, %h0}
rol{w}\t{$8, %0|%0, 8}"
...
(set (attr "preferred_for_speed")
(cond [(eq_attr "alternative" "0")
(symbol_ref "TARGET_USE_XCHGB")
(eq_attr "alternative" "1")
(symbol_ref "!TARGET_USE_XCHGB")
]
(symbol_ref "true")))])
If an alternative is a subset of some other alternative ("Q" is a subset of "r"
in the above pattern), then the compiler will *still* emit XCHG alternative 0
even if it is disabled by targeting !TARGET_USE_XCHGB target.
I will implement this functionality the hard way by providing several
target-dependant patterns. But the issue is surprising at least.
