arm: make arm_neon.h compatible with '-march=<base> -mfloat-abi=softfp'

With -mfpu set to auto, an architecture specification that lacks
floating-point, but has -mfloat-abi=softfp will cause a misleading
error.  Specifically, if we have

gcc -c test.c -mfloat-abi=softfp -march=armv7-a -mfpu=auto

where test.c contains #include <arm_neon.h>

then we get a misleading error:

test.c:11:2: error: #error "NEON intrinsics not available with the
    soft-float ABI.  Please use -mfloat-abi=softfp or -mfloat-abi=hard"

... the error message is advising us to add -mfloat-abi=softfp when we
already have it.

The difficulty is that we can't directly detect the softfp abi from
the available set of pre-defines.

Consider the options in this table, assuming -mfpu=auto:

				-mfloat-abi
			hard		softfp		soft
		   +-----------------------------------------------
 -march=armv7-a    |	*build-error*	__ARM_FP=0	__ARM_FP=0
 -march=armv7-a+fp |	__ARM_FP=12	__ARM_FP=12	__ARM_FP=0

However, for the first line, if we subsequently add
	 #pragma GCC target ("fpu=vfp")
then the value of __ARM_FP will change as follows:

				-mfloat-abi
			hard		softfp		soft
		   +-----------------------------------------------
 -march=armv7-a	   |	*build-error*	__ARM_FP=12	__ARM_FP=0
 -march=armv7-a+fp |	__ARM_FP=12	__ARM_FP=12	__ARM_FP=0

We can therefore distinguish between the soft and softfp ABIs by
temporarily forcing VFP instructions into the ISA.  If __ARM_FP is
still zero after doing this then we must be using the soft ABI.

gcc:
	* config/arm/arm_neon.h: Try harder to detect if we have
	the softfp ABI enabled.

1 files changed, 17 insertions, 0 deletions

diff --git a/gcc/config/arm/arm_neon.h b/gcc/config/arm/arm_neon.h
index 578ada8..cba50de 100644
--- a/gcc/config/arm/arm_neon.h
+++ b/gcc/config/arm/arm_neon.h
@@ -27,7 +27,21 @@
 #ifndef _GCC_ARM_NEON_H
 #define _GCC_ARM_NEON_H 1
 
+/* This header is only useful if we're compiling with -mfloat-abi=hard or
+   -mfloat-abi=softfp.  But we can't detect that directly here as the
+   compiler does not provide a pre-define for it.  However, we can check
+   whether forcing VFP will cause __ARM_FP to become defined and use that.  */
+
+#pragma GCC push_options
+#pragma GCC target ("fpu=vfp")
 #ifndef __ARM_FP
+#define __ARM_SOFT_ABI 1
+#else
+#define __ARM_SOFT_ABI 0
+#endif
+#pragma GCC pop_options
+
+#if __ARM_SOFT_ABI
 #error "NEON intrinsics not available with the soft-float ABI.  Please use -mfloat-abi=softfp or -mfloat-abi=hard"
 #else
 
@@ -21489,4 +21503,7 @@ vst4q_lane_bf16 (bfloat16_t * __a, bfloat16x8x4_t __b, const int __c)
 #pragma GCC pop_options
 
 #endif
+
+#undef __ARM_SOFT_ABI
+
 #endif