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Fix static-binary lazy FPU context allocation
Long ago static startup did not parse the auxiliary vector and therefore
could not get at any `AT_FPUCW' tag to check whether upon FPU context
allocation the kernel would use a FPU control word setting different to
that provided by the `__fpu_control' variable.  Static startup therefore
always initialized the FPU control word, forcing immediate FPU context
allocation even for binaries that otherwise never used the FPU.

As from GIT commit f8f900e static
startup supports parsing the auxiliary vector, so now it can avoid
explicit initialization of the FPU control word, just as can dynamic
startup, in the usual case where the setting written to the FPU control
word would be the same as the kernel uses.  This defers FPU context
allocation until the binary itself actually pokes at the FPU.

Note that the `AT_FPUCW' tag is usually absent from the auxiliary vector
in which case _FPU_DEFAULT is assumed to be the kernel default.
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Maciej W. Rozycki committed Sep 9, 2013
1 parent a9f5ce0 commit 95e7cf2
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Showing 8 changed files with 69 additions and 11 deletions.
22 changes: 22 additions & 0 deletions ChangeLog
@@ -1,3 +1,25 @@
2013-09-09 Maciej W. Rozycki <macro@codesourcery.com>

* csu/init-first.c (_init): Remove the !SHARED condition around
FPU control word initialization.
* elf/dl-support.c (_dl_fpu_control): New variable.
(_dl_aux_init) <AT_FPUCW>: Initialize it.
* math/test-fpucw.c [!FPU_CONTROL] (FPU_CONTROL): New macro.
(main): Replace _FPU_DEFAULT with FPU_CONTROL throughout.
* math/test-fpucw-static.c: New file.
* math/test-fpucw-ieee.c: New file.
* math/test-fpucw-ieee-static.c: New file.
* math/Makefile (tests): Add `test-fpucw-ieee' and
`$(tests-static)'.
(tests-static): New variable.
[($(build-shared),yes)] ($(addprefix $(objpfx),$(tests))): Move
dependency to...
[($(build-shared),yes)]
($(addprefix $(objpfx),$(filter-out $(tests-static),$(tests)))):
... this.
[($(build-shared),yes)] ($(addprefix $(objpfx),$(tests-static))):
New dependency.

2013-09-09 Allan McRae <allan@archlinux.org>

[BZ #15939]
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5 changes: 1 addition & 4 deletions csu/init-first.c
Expand Up @@ -61,11 +61,8 @@ _init (int argc, char **argv, char **envp)
if (!__libc_multiple_libcs)
{
/* Set the FPU control word to the proper default value if the
kernel would use a different value. (In a static program we
don't have this information.) */
#ifdef SHARED
kernel would use a different value. */
if (__fpu_control != GLRO(dl_fpu_control))
#endif
__setfpucw (__fpu_control);
}

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6 changes: 6 additions & 0 deletions elf/dl-support.c
Expand Up @@ -167,6 +167,9 @@ size_t _dl_phnum;
uint64_t _dl_hwcap __attribute__ ((nocommon));
uint64_t _dl_hwcap2 __attribute__ ((nocommon));

/* The value of the FPU control word the kernel will preset in hardware. */
fpu_control_t _dl_fpu_control = _FPU_DEFAULT;

/* This is not initialized to HWCAP_IMPORTANT, matching the definition
of _dl_important_hwcaps, below, where no hwcap strings are ever
used. This mask is still used to mediate the lookups in the cache
Expand Down Expand Up @@ -253,6 +256,9 @@ _dl_aux_init (ElfW(auxv_t) *av)
case AT_HWCAP2:
GLRO(dl_hwcap2) = (unsigned long int) av->a_un.a_val;
break;
case AT_FPUCW:
GLRO(dl_fpu_control) = av->a_un.a_val;
break;
#ifdef NEED_DL_SYSINFO
case AT_SYSINFO:
GL(dl_sysinfo) = av->a_un.a_val;
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11 changes: 7 additions & 4 deletions math/Makefile
Expand Up @@ -87,9 +87,11 @@ long-c-yes = $(calls:=l)

# Rules for the test suite.
tests = test-matherr test-fenv atest-exp atest-sincos atest-exp2 basic-test \
test-misc test-fpucw tst-definitions test-tgmath test-tgmath-ret \
bug-nextafter bug-nexttoward bug-tgmath1 test-tgmath-int \
test-tgmath2 test-powl tst-CMPLX tst-CMPLX2 test-snan
test-misc test-fpucw test-fpucw-ieee tst-definitions test-tgmath \
test-tgmath-ret bug-nextafter bug-nexttoward bug-tgmath1 \
test-tgmath-int test-tgmath2 test-powl tst-CMPLX tst-CMPLX2 test-snan \
$(tests-static)
tests-static = test-fpucw-static test-fpucw-ieee-static
# We do the `long double' tests only if this data type is available and
# distinct from `double'.
test-longdouble-yes = test-ldouble test-ildoubl
Expand Down Expand Up @@ -217,7 +219,8 @@ $(objpfx)libieee.a: $(objpfx)ieee-math.o
$(LN_S) $(<F) $(@F)

ifeq ($(build-shared),yes)
$(addprefix $(objpfx),$(tests)): $(objpfx)libm.so$(libm.so-version)
$(addprefix $(objpfx),$(filter-out $(tests-static),$(tests))): $(objpfx)libm.so$(libm.so-version)
$(addprefix $(objpfx),$(tests-static)): $(objpfx)libm.a
else
$(addprefix $(objpfx),$(tests)): $(objpfx)libm.a
endif
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1 change: 1 addition & 0 deletions math/test-fpucw-ieee-static.c
@@ -0,0 +1 @@
#include "test-fpucw-ieee.c"
24 changes: 24 additions & 0 deletions math/test-fpucw-ieee.c
@@ -0,0 +1,24 @@
/* FPU control word overridden initialization test.
Copyright (C) 2013 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */

#define FPU_CONTROL _FPU_IEEE

#include "test-fpucw.c"

/* Preempt the library's definition of `__fpu_control'. */
fpu_control_t __fpu_control = _FPU_IEEE;
1 change: 1 addition & 0 deletions math/test-fpucw-static.c
@@ -0,0 +1 @@
#include "test-fpucw.c"
10 changes: 7 additions & 3 deletions math/test-fpucw.c
Expand Up @@ -19,6 +19,10 @@
#include <fpu_control.h>
#include <stdio.h>

#ifndef FPU_CONTROL
# define FPU_CONTROL _FPU_DEFAULT
#endif

int
main (void)
{
Expand All @@ -30,11 +34,11 @@ main (void)

cw &= ~_FPU_RESERVED;

if (cw != (_FPU_DEFAULT & ~_FPU_RESERVED))
if (cw != (FPU_CONTROL & ~_FPU_RESERVED))
printf ("control word is 0x%lx but should be 0x%lx.\n",
(long int) cw, (long int) (_FPU_DEFAULT & ~_FPU_RESERVED));
(long int) cw, (long int) (FPU_CONTROL & ~_FPU_RESERVED));

return cw != (_FPU_DEFAULT & ~_FPU_RESERVED);
return cw != (FPU_CONTROL & ~_FPU_RESERVED);

#else
return 0;
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