Re: RFC: x86: Emit rsqrt with only -ffast-math

[Michael Matz <matz at suse dot de>]

Hi,

On Tue, 3 Nov 2009, Uros Bizjak wrote:

> > introduce a new flag for just the last transformation.  What do people 
> > think?
> 
> There were some problems with the patch that enabled rsqrt 
> transformation in -ffast-math, see PR34709 [1]. The root cause was 
> analysed and the fix was proposed [2], but unfortunately, the fix was 
> never applied.

That testcase explicitely checks for outputs being +-Inf and gives Inf as 
input in one case.  As you noted in your followup there [1], this is 
outside the scope of -ffinite-math.  Hence that after the change NaN is 
given is completely okay.  The testcase wasn't the root cause for 481.wrf 
breaking.  The root cause was to transform sqrt(a/b) into 1/sqrt(b/a) at 
all, which I indeed fixed (and added a testcase for already).  That 
transformation lead to the followup problem that suddenly behaviour at Inf 
mattered, when it really shouldn't.

So, I don't think we are obliged to make this testcase work as the author 
expected (it will change behaviour with my proposed patch).  It would 
unnecessarily slow down the rsqrt+NR expansion.

> The patch itself is OK with the testcase from [2], but please be 
> prepared for eventual surprises ...

So, given the above, still OK, even without that testcase?  I of course 
checked that 481.wrf doesn't break (neither the other cpu2006 or 
polyhedron benchmarks).  And regstrapped on x86_64-linux, with all 
languages+Ada.

[1] http://gcc.gnu.org/ml/gcc-patches/2008-01/msg00750.html


Ciao,
Michael.