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Clean up converttoint handling and document the semantics

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This patch currently only affects aarch64.

The roundtoint and converttoint internal functions are only called with small
values, so 32 bit result is enough for converttoint and it is a signed int
conversion so the return type is changed to int32_t.

The original idea was to help the compiler keeping the result in uint64_t,
then it's clear that no sign extension is needed and there is no accidental
undefined or implementation defined signed int arithmetics.

But it turns out gcc does a good job with inlining so changing the type has
no overhead and the semantics of the conversion is less surprising this way.
Since we want to allow the asuint64 (x + 0x1.8p52) style conversion, the top
bits were never usable and the existing code ensures that only the bottom
32 bits of the conversion result are used.

On aarch64 the neon intrinsics (which round ties to even) are changed to
round and lround (which round ties away from zero) this does not affect the
results in a significant way, but more portable (relies on round and lround
being inlined which works with -fno-math-errno).

The TOINT_SHIFT and TOINT_RINT macros were removed, only keep separate code
paths for TOINT_INTRINSICS and !TOINT_INTRINSICS.

	* sysdeps/aarch64/fpu/math_private.h (roundtoint): Use round.
	(converttoint): Use lround.
	* sysdeps/ieee754/flt-32/math_config.h (roundtoint): Declare and
	document the semantics when TOINT_INTRINSICS is set.
	(converttoint): Likewise.
	(TOINT_RINT): Remove.
	(TOINT_SHIFT): Remove.
	* sysdeps/ieee754/flt-32/e_expf.c (__expf): Remove the TOINT_RINT code
	path.
This commit is contained in:
Szabolcs Nagy 2018-07-04 12:29:29 +01:00
parent 690652882b
commit 43cfdf8f48
4 changed files with 36 additions and 19 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
ChangeLog
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@ -1,3 +1,16 @@
2018-08-10 Wilco Dijkstra <wdijkstr@arm.com>
Szabolcs Nagy <szabolcs.nagy@arm.com>
* sysdeps/aarch64/fpu/math_private.h (roundtoint): Use round.
(converttoint): Use lround.
* sysdeps/ieee754/flt-32/math_config.h (roundtoint): Declare and
document the semantics when TOINT_INTRINSICS is set.
(converttoint): Likewise.
(TOINT_RINT): Remove.
(TOINT_SHIFT): Remove.
* sysdeps/ieee754/flt-32/e_expf.c (__expf): Remove the TOINT_RINT code
path.
2018-08-10 Florian Weimer <fweimer@redhat.com> 2018-08-10 Florian Weimer <fweimer@redhat.com>
[BZ #23497] [BZ #23497]

17
sysdeps/aarch64/fpu/math_private.h
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@ -21,6 +21,8 @@
#include <fenv.h> #include <fenv.h>
#include <fpu_control.h> #include <fpu_control.h>
#include <stdint.h>
#include <math.h>
static __always_inline void static __always_inline void
libc_feholdexcept_aarch64 (fenv_t *envp) libc_feholdexcept_aarch64 (fenv_t *envp)
@ -298,25 +300,20 @@ libc_feresetround_noex_aarch64_ctx (struct rm_ctx *ctx)
#define libc_feresetround_noexf_ctx libc_feresetround_noex_aarch64_ctx #define libc_feresetround_noexf_ctx libc_feresetround_noex_aarch64_ctx
#define libc_feresetround_noexl_ctx libc_feresetround_noex_aarch64_ctx #define libc_feresetround_noexl_ctx libc_feresetround_noex_aarch64_ctx
/* Hack: only include the large arm_neon.h when needed. */ /* Use inline round and lround instructions. */
#ifdef _MATH_CONFIG_H #define TOINT_INTRINSICS 1
# include <arm_neon.h>
/* ACLE intrinsics for frintn and fcvtns instructions. */
# define TOINT_INTRINSICS 1
static inline double_t static inline double_t
roundtoint (double_t x) roundtoint (double_t x)
{ {
return vget_lane_f64 (vrndn_f64 (vld1_f64 (&x)), 0); return round (x);
} }
static inline uint64_t static inline int32_t
converttoint (double_t x) converttoint (double_t x)
{ {
return vcvtnd_s64_f64 (x); return lround (x);
} }
#endif
#include_next <math_private.h> #include_next <math_private.h>

5
sysdeps/ieee754/flt-32/e_expf.c
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@ -85,10 +85,7 @@ __expf (float x)
#if TOINT_INTRINSICS #if TOINT_INTRINSICS
kd = roundtoint (z); kd = roundtoint (z);
ki = converttoint (z); ki = converttoint (z);
#elif TOINT_RINT #else
kd = rint (z);
ki = (long) kd;
#elif TOINT_SHIFT
# define SHIFT __exp2f_data.shift # define SHIFT __exp2f_data.shift
kd = math_narrow_eval ((double) (z + SHIFT)); /* Needs to be double. */ kd = math_narrow_eval ((double) (z + SHIFT)); /* Needs to be double. */
ki = asuint64 (kd); ki = asuint64 (kd);

20
sysdeps/ieee754/flt-32/math_config.h
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@ -38,13 +38,23 @@
#endif #endif
#ifndef TOINT_INTRINSICS #ifndef TOINT_INTRINSICS
/* When set, the roundtoint and converttoint functions are provided with
the semantics documented below. */
# define TOINT_INTRINSICS 0 # define TOINT_INTRINSICS 0
#endif #endif
#ifndef TOINT_RINT
# define TOINT_RINT 0 #if TOINT_INTRINSICS
#endif /* Round x to nearest int in all rounding modes, ties have to be rounded
#ifndef TOINT_SHIFT consistently with converttoint so the results match. If the result
# define TOINT_SHIFT 1 would be outside of [-2^31, 2^31-1] then the semantics is unspecified. */
static inline double_t
roundtoint (double_t x);
/* Convert x to nearest int in all rounding modes, ties have to be rounded
consistently with roundtoint. If the result is not representible in an
int32_t then the semantics is unspecified. */
static inline int32_t
converttoint (double_t x);
#endif #endif
static inline uint32_t static inline uint32_t