[RFC] x86_64: Add SSE sfp-exceptions

[Adhemerval Zanella]

The exported x86_64 fenv.h functions operate on both i387 and SSE (since
they should work on both float, double, and long double) while the
internal libc_fe* set either SSE (float, double, and float128) or
i387 (long double).

The libgcc __sfp_handle_exceptions (used on float128 implementation),
however, will set either SEE or i387 exception depending of the
exception to raise.  This broke the internal assumption of float128
where only SSE operations will be used.

This patch reimplements the libgcc __sfp_handle_exceptions to use only
SSE operations and sets libgcc to use it instead of its own
implementation.

And I think we should fix libgcc in a similar manner, since checking on
config/i386/64/sfp-machine.h it already only supports SSE rounding mode
and x86_64 ABI also expectes float128 to use SSE registers [1]
(although it is not clear on how future implementation might implement
it).

Checked on x86_64-linux-gnu.

[1] https://github.com/hjl-tools/x86-psABI/wiki/X86-psABI
---
 sysdeps/x86/fpu/sfp-exceptions.c | 58 ++++++++++++++++++++++++++++++++
 sysdeps/x86_64/fpu/Makefile      |  3 ++
 2 files changed, 61 insertions(+)
 create mode 100644 sysdeps/x86/fpu/sfp-exceptions.c

diff --git a/sysdeps/x86/fpu/sfp-exceptions.c b/sysdeps/x86/fpu/sfp-exceptions.c
new file mode 100644
index 0000000000..2ddbfe11ba
--- /dev/null
+++ b/sysdeps/x86/fpu/sfp-exceptions.c
@@ -0,0 +1,58 @@
+/* x86_64 soft-fp exception handling for _Float128.
+   Copyright (C) 2020 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <fenv.h>
+#include <float.h>
+#include <math-barriers.h>
+#include <soft-fp.h>
+
+void
+__sfp_handle_exceptions (int _fex)
+{
+  if (_fex & FP_EX_INVALID)
+    {
+      float f = 0.0f;
+      math_force_eval (f / f);
+    }
+  if (_fex & FP_EX_DENORM)
+    {
+      float f = FLT_MIN, g = 2.0f;
+      math_force_eval (f / g);
+    }
+  if (_fex & FP_EX_DIVZERO)
+    {
+      float f = 1.0f, g = 0.0f;
+      math_force_eval (f / g);
+    }
+  if (_fex & FP_EX_OVERFLOW)
+    {
+      float force_underflow = FLT_MAX * FLT_MAX;
+      math_force_eval (force_underflow);
+    }
+  if (_fex & FP_EX_UNDERFLOW)
+    {
+      float force_overflow = FLT_MIN * FLT_MIN;
+      math_force_eval (force_overflow);
+    }
+  if (_fex & FP_EX_INEXACT)
+    {
+      float f = 1.0f, g = 3.0f;
+      math_force_eval (f / g);
+    }
+}
+strong_alias (__sfp_handle_exceptions, __wrap___sfp_handle_exceptions)
diff --git a/sysdeps/x86_64/fpu/Makefile b/sysdeps/x86_64/fpu/Makefile
index a4ff2723a8..5becb96fa3 100644
--- a/sysdeps/x86_64/fpu/Makefile
+++ b/sysdeps/x86_64/fpu/Makefile
@@ -25,6 +25,9 @@ endif
 
 # Variables for libmvec tests.
 ifeq ($(subdir),math)
+libm-routines += sfp-exceptions
+LDFLAGS-m.so += -Wl,--wrap=__sfp_handle_exceptions
+
 ifeq ($(build-mathvec),yes)
 libmvec-tests += double-vlen2 double-vlen4 double-vlen4-avx2 \
 		 float-vlen4 float-vlen8 float-vlen8-avx2
-- 
2.17.1