diff mbox

[v2,1/3] Cleanup __ieee754_sqrt(f/l)

Message ID DB6PR0801MB205353FC2770C2248701BF1483020@DB6PR0801MB2053.eurprd08.prod.outlook.com
State New, archived
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Commit Message

Wilco Dijkstra Dec. 22, 2017, 3:23 p.m. UTC
Joseph Myers wrote:

> float128_private.h isn't an external header.  It includes math.h and other 
> headers, then redefines symbols from them, and is included before the main 
> implementation of a float128 function.  Naturally your problem needs 
> debugging, since of course libm should be exporting sqrtf128.  (And 
> float128_private.h shouldn't be included at all when the type-generic 
> wrappers are, so shouldn't have any opportunity to cause trouble with the 
> automatically generated w_sqrtf128 which is what ought to be defining 
> sqrtf128 as an alias to __sqrtf128.)

Well it seems the issue is all the autogenerated magic templates. It appears
they are used to both define simple wrappers as well as complex float128
functions which happen to use sqrt (in the form of M_SQRT). For the former
you want to disable the asm redirect, for the latter you must redirect it. Given
they are all generated in the same way, it's impossible to do the right thing.
So I don't think we can change M_SQRT until the wrapper magic has been
removed or at least greatly simplified.

Here is the latest version, which adds redirect for float128 but keeps M_SQRT
as is - this passes on x86, x64, ppc, aarch64:


This patch series cleans up the many uses of  __ieee754_sqrt(f/l) in GLIBC.
The goal is to enable GCC to do the inlining, and if this fails call the
__ieee754_sqrt function.  This is done by internally declaring sqrt with asm
redirects.  The compat symbols and sqrt wrappers need to disable the redirect.
The redirect is also disabled if there are already redirects defined when
using -ffinite-math-only.

This means targets are no longer forced to add a special inline for sqrt.

All math functions (but not math tests) are built with -fno-math-errno which
means GCC will typically inline sqrt as a single instruction.


ChangeLog:
2017-12-22  Wilco Dijkstra  <wdijkstr@arm.com>

        * math/Makefile: Emit NO_MATH_REDIRECT in wrapper templates.
        * include/math.h (sqrt): Declare with asm redirect.
        (sqrtf): Likewise.
        (sqrtl): Likewise.
        * Makeconfig: Add -fno-math-errno for libc/libm, but build tests
        with -fmath-errno.
        * math/w_sqrt_compat.c: Define NO_MATH_REDIRECT.
        * math/w_sqrt_template.c: Likewise.
        * math/w_sqrtf_compat.c: Likewise.
        * math/w_sqrtl_compat.c: Likewise.
        * sysdeps/i386/fpu/w_sqrt.c: Likewise.
        * sysdeps/i386/fpu/w_sqrt_compat.c: Likewise.
--

Comments

Joseph Myers Dec. 22, 2017, 5:03 p.m. UTC | #1
On Fri, 22 Dec 2017, Wilco Dijkstra wrote:

> Joseph Myers wrote:
> 
> > float128_private.h isn't an external header.  It includes math.h and other 
> > headers, then redefines symbols from them, and is included before the main 
> > implementation of a float128 function.  Naturally your problem needs 
> > debugging, since of course libm should be exporting sqrtf128.  (And 
> > float128_private.h shouldn't be included at all when the type-generic 
> > wrappers are, so shouldn't have any opportunity to cause trouble with the 
> > automatically generated w_sqrtf128 which is what ought to be defining 
> > sqrtf128 as an alias to __sqrtf128.)
> 
> Well it seems the issue is all the autogenerated magic templates. It appears
> they are used to both define simple wrappers as well as complex float128
> functions which happen to use sqrt (in the form of M_SQRT). For the former
> you want to disable the asm redirect, for the latter you must redirect it. Given
> they are all generated in the same way, it's impossible to do the right thing.
> So I don't think we can change M_SQRT until the wrapper magic has been
> removed or at least greatly simplified.

"impossible to do the right thing" does not make sense here.  Whether 
redirections should be disabled is a function of the particular template 
in question, and so should be handled through the templates that want to 
disable redirection defining NO_MATH_REDIRECT and those that don't not 
defining it.  Defining it globally in the Makefile as this patch version 
does is obviously wrong.

Is your problem actually that math-type-macros-float128.h, unlike the 
other math-type-macros-*.h files, includes <math.h> and <complex.h>, 
meaning they get included before any code from the template has a chance 
to define NO_MATH_REDIRECT?  If so, I'd suggest removing those includes 
and seeing if that helps.

Now, maybe it's not enough in all cases: math-type-macros-double.h 
includes libm-alias-double.h, of which the ldbl-opt version includes 
math_ldbl_opt.h which includes math.h, and likewise for ldouble.  So it 
would also be important to look at sqrt / sqrtl wrappers in *static* libm 
for ldbl-opt configurations (as for most configurations, only static libm 
is using the type-generic wrappers rather than the compat ones - but we 
don't have good testsuite coverage for whether static libraries provide 
the intended API, so missing symbols in them could easily not result in 
any test failures).

I'm not sure anything ought to be depending on math_ldbl_opt.h including 
math.h and math_private.h either; quite possibly those includes could also 
be safely removed, maybe adding explicit includes to any source files that 
were relying on them (though at that point I think you're in the territory 
of wanting before-and-after comparison of stripped installed shared 
libraries from build-many-glibcs.py --strip, to make sure there are no 
changes to installed libraries at all).

But while I think such include removals are a sensible cleanup, relying on 
not having any indirect inclusion path to math.h from math-type-macros-*.h 
is also fragile, and I don't think it's necessary.  Rather, you could have 
a mechanism for templates to cause NO_MATH_REDIRECT to be defined before 
the automatic inclusion of math-type-macros-*.h.  For example, have the 
math/Makefile template logic generate such a define for functions in 
a makefile variable $(gen-calls-no-math-redirect).  If you do something 
like that, you don't need to remove any includes.  And you also avoid the 
clear incorrectness of defining NO_MATH_REDIRECT for all templates rather 
than only particular cases needing it.

(Once you have such makefile logic of course you no longer define 
NO_MATH_REDIRECT directly in w_sqrt_template.c.)
Joseph Myers Dec. 23, 2017, 5:07 p.m. UTC | #2
Another observation on the use of -fno-math-errno for building libm:

ldbl-opt configurations build libnldbl_nonshared.a, a library of wrappers 
such as nldbl-sqrt.c:

#include "nldbl-compat.h"

double
attribute_hidden
sqrtl (double x)
{
  return sqrt (x);
}

These are intended for use with compilers that are (a) using 64-bit long 
double in such a configuration and (b) do not support asm redirection, so 
that a call to sqrtl actually ends up calling sqrtl in the object file and 
so needs such a wrapper to end up calling the double function (whereas 
with asm redirection support, math.h arranges for long double function 
calls to be redirected to call the double functions).  Thus, this sqrtl 
wrapper needs to call the public sqrt function, both without inlining that 
assumes no-math-errno and without any redirection of either sqrtl or sqrt 
to corresponding __ieee754_* names.  So it needs to disable redirection, 
*and* the whole of libnldbl_nonshared.a needs to be built with 
-fmath-errno because it's meant to be calling public errno-setting 
functions.  And when other libm functions such as floor get redirection, 
the libnldbl_nonshared.a versions of those need to disable it as well (not 
because of any errno setting issues, but because nldbl-floor.c needs to 
provide the public symbol floorl, and to call the symbol floor, if not 
inlined, rather than the symbol __floor which is not exported from 
libm.so).

(It may be that we should declare support for asm redirection a required 
feature for compilers used with installed glibc headers, which would both 
allow some header cleanup and allow removing libnldbl_nonshared.a - I 
think it makes sense to have an official list of required compiler 
features, beyond C90/C++98, and to include this on that list, and suspect 
that actually compilers without asm redirection support already don't work 
with installed headers, generally or in this particular case.  But while 
we haven't declared that a required feature, certain libm changes need to 
allow for libnldbl_nonshared.a.)
diff mbox

Patch

diff --git a/Makeconfig b/Makeconfig
index 34bed9790fac1fd0be9e16b7fe2fa6f5c479b32b..b9da1c137bc495a2f63064765225dc8878cde306 100644
--- a/Makeconfig
+++ b/Makeconfig
@@ -831,6 +831,9 @@  endif
 # disable any optimization that assume default rounding mode.
 +math-flags = -frounding-math
 
+# Build libc/libm using -fno-math-errno, but run testsuite with -fmath-errno.
++extra-math-flags = $(if $(filter nonlib testsuite,$(in-module)),-fmath-errno,-fno-math-errno)
+
 # We might want to compile with some stack-protection flag.
 ifneq ($(stack-protector),)
 +stack-protector=$(stack-protector)
@@ -966,6 +969,7 @@  endif
 
 override CFLAGS	= -std=gnu11 -fgnu89-inline $(config-extra-cflags) \
 		  $(filter-out %frame-pointer,$(+cflags)) $(+gccwarn-c) \
+		  $(+extra-math-flags) \
 		  $(sysdep-CFLAGS) $(CFLAGS-$(suffix $@)) $(CFLAGS-$(<F)) \
 		  $(CFLAGS-$(@F)) $(tls-model) \
 		  $(foreach lib,$(libof-$(basename $(@F))) \
diff --git a/include/math.h b/include/math.h
index 7ee291fc972088a1409949326f7024e8e3fe5415..c25033fcd5add2f4067fdbb51fb345056d6a1400 100644
--- a/include/math.h
+++ b/include/math.h
@@ -54,5 +54,20 @@  libm_hidden_proto (__expf128)
 libm_hidden_proto (__expm1f128)
 # endif
 
+# if !(defined __FINITE_MATH_ONLY__ && __FINITE_MATH_ONLY__ > 0)
+#  ifndef NO_MATH_REDIRECT
+/* Declare sqrt for use within GLIBC.  Sqrt will typically inline into a
+   single instruction.  Use an asm to avoid use of PLTs if it doesn't.  */
+float (sqrtf) (float) asm ("__ieee754_sqrtf");
+double (sqrt) (double) asm ("__ieee754_sqrt");
+#   ifndef __NO_LONG_DOUBLE_MATH
+long double (sqrtl) (long double) asm ("__ieee754_sqrtl");
+#   endif
+#   if __HAVE_DISTINCT_FLOAT128 > 0
+_Float128 (sqrtf128) (_Float128) asm ("__ieee754_sqrtf128");
+#   endif
+#  endif
+# endif
+
 #endif
 #endif
diff --git a/math/Makefile b/math/Makefile
index 8978f2ef963e67acb4e3d252733cdb0e19cf6921..f9b083d8c4049bdfdaaff0a78aa26139ace83bf5 100644
--- a/math/Makefile
+++ b/math/Makefile
@@ -397,6 +397,7 @@  $(objpfx)gen-libm-templates.stmp: Makefile
 	    type=$${type%__*};                                            \
 	    file=$(objpfx)$${gcall%F*}$${suff}$${gcall#*F}.c;             \
 	    (                                                             \
+	      echo "#define NO_MATH_REDIRECT";				  \
 	      echo "#include <math-type-macros-$${type}.h>";              \
 	      echo "#include <$${func}_template.c>";                      \
 	    ) > $${file};                                                 \
diff --git a/math/w_sqrt_compat.c b/math/w_sqrt_compat.c
index 3280d2fbb86af2aeaf6e686fd38579b209c901aa..1a8db2fdef87e57df2f7dda117ef3ee48581ca1d 100644
--- a/math/w_sqrt_compat.c
+++ b/math/w_sqrt_compat.c
@@ -16,6 +16,7 @@ 
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
+#define NO_MATH_REDIRECT
 #include <math.h>
 #include <math_private.h>
 #include <math-svid-compat.h>
diff --git a/math/w_sqrt_template.c b/math/w_sqrt_template.c
index 5fae302382d10e9b05df2665c9cb05126cd62f02..f03f04751a9334fc7cdb8c954fbbcd845ac8670f 100644
--- a/math/w_sqrt_template.c
+++ b/math/w_sqrt_template.c
@@ -21,6 +21,7 @@ 
    for each floating-point type.  */
 #if __USE_WRAPPER_TEMPLATE
 
+# define NO_MATH_REDIRECT
 # include <errno.h>
 # include <fenv.h>
 # include <math.h>
diff --git a/math/w_sqrtf_compat.c b/math/w_sqrtf_compat.c
index 6c8c7e3857c7dacf52de6b575a2386095688b5dc..fe7418dbdcdbdb571cb1b9743cf9b17eaa5c8cb2 100644
--- a/math/w_sqrtf_compat.c
+++ b/math/w_sqrtf_compat.c
@@ -16,6 +16,7 @@ 
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
+#define NO_MATH_REDIRECT
 #include <math.h>
 #include <math_private.h>
 #include <math-svid-compat.h>
diff --git a/math/w_sqrtl_compat.c b/math/w_sqrtl_compat.c
index 0590f6d155fee27d41bfd42c1dbdf19c381ba1c8..3fda9cf991e430f259b9d26f15219181b3e6dcbb 100644
--- a/math/w_sqrtl_compat.c
+++ b/math/w_sqrtl_compat.c
@@ -16,6 +16,7 @@ 
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
+#define NO_MATH_REDIRECT
 #include <math.h>
 #include <math_private.h>
 #include <math-svid-compat.h>
diff --git a/sysdeps/i386/fpu/w_sqrt.c b/sysdeps/i386/fpu/w_sqrt.c
index d37a5d55bf439cebdff05f05dd66f910d6d48c18..8bef04e68a7be3d21d89e6eb41dcfa2561f3f6a5 100644
--- a/sysdeps/i386/fpu/w_sqrt.c
+++ b/sysdeps/i386/fpu/w_sqrt.c
@@ -1,5 +1,6 @@ 
 /* The inline __ieee754_sqrt is not correctly rounding; it's OK for
    most internal uses in glibc, but not for sqrt itself.  */
+#define NO_MATH_REDIRECT
 #define __ieee754_sqrt __avoid_ieee754_sqrt
 #include <math.h>
 #include <math_private.h>
diff --git a/sysdeps/i386/fpu/w_sqrt_compat.c b/sysdeps/i386/fpu/w_sqrt_compat.c
index ddd36d0964c2945579bc59b02f127af88384acca..dd485f4b88c7f5348abaab81a1308aeadc9594ec 100644
--- a/sysdeps/i386/fpu/w_sqrt_compat.c
+++ b/sysdeps/i386/fpu/w_sqrt_compat.c
@@ -1,5 +1,6 @@ 
 /* The inline __ieee754_sqrt is not correctly rounding; it's OK for
    most internal uses in glibc, but not for sqrt itself.  */
+#define NO_MATH_REDIRECT
 #define __ieee754_sqrt __avoid_ieee754_sqrt
 #include <math.h>
 #include <math_private.h>