[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH for-4.19] x86/altcall: fix clang code-gen when using altcall in loop constructs
On Wed, Jul 24, 2024 at 11:16:52AM +0200, Jan Beulich wrote: > On 24.07.2024 11:13, Roger Pau Monné wrote: > > On Wed, Jul 24, 2024 at 10:41:43AM +0200, Jan Beulich wrote: > >> On 24.07.2024 10:28, Roger Pau Monné wrote: > >>> The only way I've found to cope with this is to use something like: > >>> > >>> #define ALT_CALL_ARG(arg, n) \ > >>> union { \ > >>> typeof(arg) e[(sizeof(long) + sizeof(arg) - 1) / sizeof(arg)]; \ > >>> unsigned long r; \ > >>> } a ## n ## __ = { \ > >>> .e[0] = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); })\ > >>> }; \ > >>> register unsigned long a ## n ## _ asm ( ALT_CALL_arg ## n ) = \ > >>> a ## n ## __.r > >>> > >>> An oversized array that ensures all the space of the long is covered > >>> by the array, but then we need an extra variable, as we would > >>> otherwise require modifying ALT_CALL{0..6}_OUT to use aX_.r instead of > >>> aX_. > >> > >> I don't think we need to over-size the array. It just gets yet a little > >> more complex then: > >> > >> #define ALT_CALL_ARG(arg, n) \ > >> register union { \ > >> struct { \ > >> typeof(arg) e; \ > >> char pad[sizeof(long) - sizeof(arg)]; \ > >> } s; \ > >> unsigned long r; \ > >> } a ## n ## _ asm ( ALT_CALL_arg ## n ) = { \ > >> .s.e = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); }) \ > >> } > > > > We could even simplify this, there's no need for the union anymore, > > since struct size == sizeof(long) already: > > > > #define ALT_CALL_ARG(arg, n) \ > > register struct { \ > > typeof(arg) e; \ > > char pad[sizeof(long) - sizeof(arg)]; \ > > } a ## n ## _ asm ( ALT_CALL_arg ## n ) = { \ > > .e = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); }) \ > > } > > > > The above seems to work for both the original issue and the new one. > > Oh, good. If the compiler's happy with not being dealt an unsigned long, > even better. Maybe for consistency we want to use sizeof(void *) also in > pad's definition then. I have to test this on Gitlab, it seems to be fine with clang version 18.1.6, not sure about older releases. I can switch to void *, that would be fine, will attempt to prepare a followup patch. Thanks, Roger.
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