Date: Sat, 13 May 2017 13:00:59 +0200 From: Dimitry Andric <dimitry@andric.com> To: sgk@troutmask.apl.washington.edu Cc: numerics@freebsd.org, freebsd-hackers@freebsd.org Subject: Re: catrig[fl].c and inexact Message-ID: <42D3F536-42D7-4097-A500-0EF939584592@andric.com> In-Reply-To: <20170512215654.GA82545@troutmask.apl.washington.edu> References: <20170512215654.GA82545@troutmask.apl.washington.edu>
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On 12 May 2017, at 23:56, Steve Kargl <sgk@troutmask.apl.washington.edu> =
wrote:
>=20
> So, I've been making improvements to my implementations of
> the half-cycle trig functions. In doing so, I decide to
> add WARNS=3D2 to msun/Makefile. clang 4.0.0 dies with an
> error about an unused variable in raise_inexact() from
> catrig[fl].c.
>=20
> /usr/home/kargl/trunk/math/libm/msun/src/catrigl.c:195:2: error: =
unused variable
> 'junk' [-Werror,-Wunused-variable]
> raise_inexact();
> ^
> /usr/home/kargl/trunk/math/libm/msun/src/catrigl.c:56:45: note: =
expanded from
> macro 'raise_inexact'
> #define raise_inexact() do { volatile float junk =3D 1 + tiny; } =
while(0)
> ^
> Grepping catrig.o for the variable 'junk' suggests that 'junk' is
> optimized out (with at least -O2).
As far as I can see, this is not the case. The simplest reduction is
this:
static const volatile float tiny =3D 0x1p-100;
void f(void)
{
volatile float junk =3D 1 + tiny;
}
For i386-freebsd, this results in the following (boilerplate left out):
$ clang-4.0.0 -target i386-freebsd -O2 -S vol1.c -o -
[...]
pushl %ebp
movl %esp, %ebp
pushl %eax
fld1
fadds tiny
fstps -4(%ebp)
addl $4, %esp
popl %ebp
retl
[...]
tiny:
.long 226492416 # float 7.88860905E-31
For amd64-freebsd:
$ clang-4.0.0 -target amd64-freebsd -O2 -S vol1.c -o -
[...]
.LCPI0_0:
.long 1065353216 # float 1
[...]
pushq %rbp
movq %rsp, %rbp
movss tiny(%rip), %xmm0 # xmm0 =3D mem[0],zero,zero,zero
addss .LCPI0_0(%rip), %xmm0
movss %xmm0, -4(%rbp)
popq %rbp
retq
[...]
tiny:
.long 226492416 # float 7.88860905E-31
I also tried -O3, but it doesn't change the result.
-Dimitry
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