Re: [PATCH] implement asin() and acos() as built-in x87 function

[Roger Sayle <roger at eyesopen dot com>]

On Tue, 20 Apr 2004, Uros Bizjak wrote:
> 2004-04-20  Uros Bizjak  <uros@kss-loka.si>
>
>     * optabs.h (enum optab_index): Add new OTI_asin and OTI_acos.
>     (asin_optab, acos_optab): Define corresponding macros.
>     * optabs.c (init_optabs): Initialize asin_optab and acos_optab.
>     * genopinit.c (optabs): Implement asin_optab and acos_optab
>     using asin?f2 and acos?f2 patterns.
>     * builtins.c (expand_builtin_mathfn): Handle BUILT_IN_ASIN{,F,L}
>     using asin_optab, and BUILT_IN_ACOS{,F,L} using acos_optab.
>     (expand_builtin): Expand BUILT_IN_ASIN{,F,L} and BUILT_IN_ACOS{,F,L}
>     using expand_builtin_mathfn if flag_unsafe_math_optimizations is set.
>
>     * config/i386/i386.md (asindf2, asinsf2, asinxf2, acosdf2,
>     acossf2, acosxf2): New expanders to implement asin, asinf, asinl,
>     acos, acosf and acosl built-ins as inline x87 intrinsics.
>
>     * gcc.dg/builtins-39.c: New test.

This is OK for mainline.  I've committed it to CVS after confirming
there are no testsuite regressions following an i686-pc-linux-gnu
bootstrap.

Thanks.


> BTW (for bonus points ;-) : calculating both asin() and acos() of the
> same parameter produces quite optimized code. Optimizers detect, that
> only one parameter load, one multiplication and one call to fsqrt is
> needed:

Here's another nice one.

	double pi = acos(-1.0);

evaluates the square, addition and square root at compile-time, and
generates the inline equivalent of "double pi = atan2(0.0,-1.0)".


Richard Guenther's points on optimize_size are well taken.  We need
to devise a "model system" where we can evaluate the size differences
of these intrinsics as used in a typical environment.  For example,
comparing "double foo(double x) { return acos(x); }" with and without
-Os is a bad metric, as GCC will optimize this code into a tailcall
which is unrepresentative of most math function call sites.  Similarly
concerns over call-clobbered spills, availability of operands in FP
registers, etc...  Once we have a suitable measure, we can evaluate
and correct all of GCC's x87 intrinsics at the same time.

Roger
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