[2/3] Change the return type of converttoint and document the, semantics

Message ID 647d7376-fa40-4f75-d580-8c8f957d8e05@arm.com
State Superseded
Headers show
Series
  • Updates to the new math code
Related show

Commit Message

Szabolcs Nagy July 4, 2018, 3:53 p.m.

Patch

From 99ee950a35cdca2f19ac0b9692d4ad8077cdaad9 Mon Sep 17 00:00:00 2001
From: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date: Wed, 4 Jul 2018 11:09:39 +0100
Subject: [PATCH 2/3] Change the return type of converttoint and document the
 semantics

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 natural return type is 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.

In newlib with default settings only aarch64 is affected and there is no
significant code generation change with gcc after the patch.
---
 newlib/libm/common/math_config.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/newlib/libm/common/math_config.h b/newlib/libm/common/math_config.h
index aec9cd0d6..220ebd6ef 100644
--- a/newlib/libm/common/math_config.h
+++ b/newlib/libm/common/math_config.h
@@ -62,15 +62,23 @@ 
 #endif
 
 #if HAVE_FAST_ROUND
+/* When set, the roundtoint and converttoint functions are provided with
+   the semantics documented below.  */
 # define TOINT_INTRINSICS 1
 
+/* Round x to nearest int in all rounding modes, ties have to be rounded
+   consistently with converttoint so the results match.  If the result
+   would be outside of [-2^31, 2^31-1] then the semantics is unspecified.  */
 static inline double_t
 roundtoint (double_t x)
 {
   return round (x);
 }
 
-static inline uint64_t
+/* 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)
 {
 # if HAVE_FAST_LROUND
-- 
2.14.1