Re: [PATCH] MIPS: Loongson, workaround ll/sc weak ordering

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On Fri, 14 Dec 2018, Paul Burton wrote:

> > We provides a config-time option for gcc. So whether
> > fix-loongson3-llsc option will be default,
> > is not sure. It depends on distributions.
> 
> OK but even if it's default on, will that only apply when we provide
> -march=loongson3a? Surely it makes no sense to default on for other
> CPUs.

 GCC defaults chosen at the configuration time apply regardless of any 
`-march=' command-line option used with a given compilation (of course 
unless the architecture selected actually precludes the use of the 
affected instructions or code sequences), and I think it does make sense, 
as this way you can have a compiler that defaults to some generic 
architecture with all the workarounds enabled, which produces code safe to 
run anywhere.

 I could envisage an option like `-mfix-loongson3-llsc=from-arch' or a 
broader `-mfix=from-arch' one, and a corresponding set of GCC defaults to 
choose from at the configuration time, however generic support for that 
hasn't been implemented.  You could file an enhancement request with GCC 
bugzilla to have a MIPS GCC port maintainer look into it I suppose.  It 
should be moderately easy to implement I believe.

 IIRC not all `-mfix-foo' command-line options have a corresponding GCC
configuration option, in which case they default to off.

 HTH,

  Maciej


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