[PATCH v3 6/7] i386: Move hypot implementation to C

[PATCH v3 6/7] i386: Move hypot implementation to C

[Adhemerval Zanella via Libc-alpha]

The generic hypotf is slight slower, mostly due the tricks the assembly
does to optimize the isinf/isnan/issignaling.  The generic hypot is way
slower, since the optimized implementation uses the i386 default
excessive precision to issue the operation directly.  A similar
implementation is provided instead of using the generic implementation:

Checked on i686-linux-gnu.
---
 sysdeps/i386/fpu/e_hypot.S  | 75 -------------------------------------
 sysdeps/i386/fpu/e_hypot.c  | 46 +++++++++++++++++++++++
 sysdeps/i386/fpu/e_hypotf.S | 64 -------------------------------
 3 files changed, 46 insertions(+), 139 deletions(-)
 delete mode 100644 sysdeps/i386/fpu/e_hypot.S
 create mode 100644 sysdeps/i386/fpu/e_hypot.c
 delete mode 100644 sysdeps/i386/fpu/e_hypotf.S

diff --git a/sysdeps/i386/fpu/e_hypot.S b/sysdeps/i386/fpu/e_hypot.S
deleted file mode 100644
index f2c956b77a..0000000000
--- a/sysdeps/i386/fpu/e_hypot.S
+++ /dev/null
@@ -1,75 +0,0 @@
-/* Compute the hypothenuse of X and Y.
-   Copyright (C) 1998-2021 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 <sysdep.h>
-#include <i386-math-asm.h>
-#include <libm-alias-finite.h>
-
-DEFINE_DBL_MIN
-
-#ifdef PIC
-# define MO(op) op##@GOTOFF(%edx)
-#else
-# define MO(op) op
-#endif
-
-	.text
-ENTRY(__ieee754_hypot)
-#ifdef  PIC
-	LOAD_PIC_REG (dx)
-#endif
-	fldl	4(%esp)		// x
-	fxam
-	fnstsw
-	fldl	12(%esp)	// y : x
-	movb	%ah, %ch
-	fxam
-	fnstsw
-	movb	%ah, %al
-	orb	%ch, %ah
-	sahf
-	jc	1f
-	fmul	%st(0)		// y * y : x
-	fxch			// x : y * y
-	fmul	%st(0)		// x * x : y * y
-	faddp			// x * x + y * y
-	fsqrt
-	DBL_NARROW_EVAL_UFLOW_NONNEG
-2:	ret
-
-	// We have to test whether any of the parameters is Inf.
-	// In this case the result is infinity.
-1:	andb	$0x45, %al
-	cmpb	$5, %al
-	je	3f		// jump if y is Inf
-	andb	$0x45, %ch
-	cmpb	$5, %ch
-	jne	4f		// jump if x is not Inf
-	fxch
-3:	fstp	%st(1)
-	fabs
-	jmp	2b
-
-4:	testb	$1, %al
-	jnz	5f		// y is NaN
-	fxch
-5:	fstp	%st(1)
-	jmp	2b
-
-END(__ieee754_hypot)
-libm_alias_finite (__ieee754_hypot, __hypot)
diff --git a/sysdeps/i386/fpu/e_hypot.c b/sysdeps/i386/fpu/e_hypot.c
new file mode 100644
index 0000000000..f2c9a8f1b9
--- /dev/null
+++ b/sysdeps/i386/fpu/e_hypot.c
@@ -0,0 +1,46 @@
+/* Euclidean distance function.  Double/Binary64 i386 version.
+   Copyright (C) 2021 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 <math.h>
+#include <math_private.h>
+#include <math-underflow.h>
+#include <math-narrow-eval.h>
+#include <libm-alias-finite.h>
+#include <math_config.h>
+
+/* The i386 allows to use the default excess of precision to optimize the
+   hypot implementation, since internal multiplication and sqrt is carried
+   with 80-bit FP type.  */
+double
+__ieee754_hypot (double x, double y)
+{
+  if (!isfinite (x) || !isfinite (y))
+    {
+      if ((isinf (x) || isinf (y))
+	  && !issignaling (x) && !issignaling (y))
+	return INFINITY;
+      return x + y;
+    }
+
+  long double lx = x;
+  long double ly = y;
+  double r = sqrtl (math_narrow_eval (lx * lx + ly * ly));
+  math_check_force_underflow_nonneg (r);
+  return r;
+}
+libm_alias_finite (__ieee754_hypot, __hypot)
diff --git a/sysdeps/i386/fpu/e_hypotf.S b/sysdeps/i386/fpu/e_hypotf.S
deleted file mode 100644
index cec5d15403..0000000000
--- a/sysdeps/i386/fpu/e_hypotf.S
+++ /dev/null
@@ -1,64 +0,0 @@
-/* Compute the hypothenuse of X and Y.
-   Copyright (C) 1998-2021 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 <sysdep.h>
-#include <i386-math-asm.h>
-#include <libm-alias-finite.h>
-
-	.text
-ENTRY(__ieee754_hypotf)
-	flds	4(%esp)		// x
-	fxam
-	fnstsw
-	flds	8(%esp)		// y : x
-	movb	%ah, %ch
-	fxam
-	fnstsw
-	movb	%ah, %al
-	orb	%ch, %ah
-	sahf
-	jc	1f
-	fmul	%st(0)		// y * y : x
-	fxch			// x : y * y
-	fmul	%st(0)		// x * x : y * y
-	faddp			// x * x + y * y
-	fsqrt
-	FLT_NARROW_EVAL
-2:	ret
-
-	// We have to test whether any of the parameters is Inf.
-	// In this case the result is infinity.
-1:	andb	$0x45, %al
-	cmpb	$5, %al
-	je	3f		// jump if y is Inf
-	andb	$0x45, %ch
-	cmpb	$5, %ch
-	jne	4f		// jump if x is not Inf
-	fxch
-3:	fstp	%st(1)
-	fabs
-	jmp	2b
-
-4:	testb	$1, %al
-	jnz	5f		// y is NaN
-	fxch
-5:	fstp	%st(1)
-	jmp	2b
-
-END(__ieee754_hypotf)
-libm_alias_finite (__ieee754_hypotf, __hypotf)
-- 
2.32.0

[PATCH v3 6/7] i386: Move hypot implementation to C

[Wilco Dijkstra via Libc-alpha]

Hi Adhemerval,

One more comment on this version:

+  long double lx = x;
+  long double ly = y;
+  double r = sqrtl (math_narrow_eval (lx * lx + ly * ly));
+  math_check_force_underflow_nonneg (r);
+  return r;

I think Joseph meant something like:

double r = math_narrow_eval ((double) sqrtl (lx * lx + ly * ly));

The narrow eval macro forces the result to memory to remove excess
precision but it only works if it has the narrower type already.

Cheers,
Wilco