[PATCH] Aarch32/64: Support __FLT_EVAL_METHOD__ values other than 0, 1, 2

[Andrea Corallo]

Hi all,

when _Float16 in native precision is supported GCC defines
__FLT_EVAL_METHOD__ to 16 as per ISO/IEC TS 18661-3.

Ex:

$ ./arm-none-eabi-gcc -mthumb -march=armv8.1-m.main+fp  -mfloat-abi=softfp -xc /dev/null -E -dM | grep FLT_EVAL
#define __FLT_EVAL_METHOD__ 16
#define __FLT_EVAL_METHOD_TS_18661_3__ 16

Unfortunately libc/include/math.h understands only values 0, 1 and 2
leading to have newlib non compilable for those configurations.

I think in newlib we should either:

1- Handle __FLT_EVAL_METHOD__ == 16
2- Build in a C mode which does not enable _Float16 (ex -std=c11)
3- Build without the _FloatN extensions -fpermitted-flt-eval-methods=c11

I think 1 should be the favorite approach.

ISO/IEC 9899:201x [1] suggests that for other values of FLT_EVAL_METHOD
other than 0, 1, 2 the definition of float_t and double_t is
implementation-defined.

The suggested patch implements that for arm and Aarch64.

Note, some previous discussion on this topic can be found here [2] as
follow-up to a patch addressing the same issue (only for the
case__FLT_EVAL_METHOD__ == 16 case).

Best Regards

  Andrea

[1] <http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf>
[2] <https://sourceware.org/legacy-ml/newlib/2019/msg00597.html>


-------------- next part --------------
>From d4c38a902e7bd5a2686d4d758c5cd563069c5bb2 Mon Sep 17 00:00:00 2001
From: Andrea Corallo <andrea.corallo@arm.com>
Date: Wed, 30 Mar 2022 15:40:59 +0200
Subject: [PATCH] Aarch32/64: Support __FLT_EVAL_METHOD__ values other
 than 0, 1, 2

2022-03-30  Andrea Corallo  <andrea.corallo@arm.com>

	* libc/include/machine/ieeefp.h (__FLOAT_TYPE, __DOUBLE_TYPE): New
	macros.
	* libc/include/math.h: Uses __DOUBLE_TYPE __FLOAT_TYPE to define
	double_t float_t if possible.
---
 newlib/libc/include/machine/ieeefp.h | 4 ++++
 newlib/libc/include/math.h           | 9 +++++++++
 2 files changed, 13 insertions(+)

diff --git a/newlib/libc/include/machine/ieeefp.h b/newlib/libc/include/machine/ieeefp.h
index 4dc13828c..c65c67769 100644
--- a/newlib/libc/include/machine/ieeefp.h
+++ b/newlib/libc/include/machine/ieeefp.h
@@ -90,6 +90,8 @@
 #ifndef __SOFTFP__
 # define _SUPPORTS_ERREXCEPT
 #endif
+#define __DOUBLE_TYPE double
+#define __FLOAT_TYPE float
 #endif

 #if defined (__aarch64__)
@@ -102,6 +104,8 @@
 #ifdef __ARM_FP
 # define _SUPPORTS_ERREXCEPT
 #endif
+#define __DOUBLE_TYPE double
+#define __FLOAT_TYPE float
 #endif

 #ifdef __epiphany__
diff --git a/newlib/libc/include/math.h b/newlib/libc/include/math.h
index ba1a8a17e..da056b5b6 100644
--- a/newlib/libc/include/math.h
+++ b/newlib/libc/include/math.h
@@ -158,6 +158,15 @@ extern int isnan (double);
    #else
     /* Implementation-defined.  Assume float_t and double_t have been
      * defined previously for this configuration (e.g. config.h). */
+
+   /* If __DOUBLE_TYPE is defined (__FLOAT_TYPE is then supposed to be
+      defined as well) float_t and double_t definition is suggested by
+      an arch specific header.  */
+   #ifdef __DOUBLE_TYPE
+    typedef __DOUBLE_TYPE double_t
+    typedef __FLOAT_TYPE float_t
+   #endif
+   /* Assume config.h has provided these types.  */
   #endif
 #else
     /* Assume basic definitions.  */
--
2.25.1