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vfma is available in the vfpv4 fpu and above, the ACLE standard feature
test for double precision hardware fma support is
__ARM_FEATURE_FMA && __ARM_FP&8
we need further checks to work around clang bugs (fixed in clang >=7.0)
&& !__SOFTFP__
because __ARM_FP is defined even with -mfloat-abi=soft
&& !BROKEN_VFP_ASM
to disable the single precision code when inline asm handling is broken.
For runtime selection the HWCAP_ARM_VFPv4 hwcap flag can be used, but
that requires further work.
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commit e4355bd6bec89688e8c739cd7b4c76e675643dca moved the math asm
from external source files to inline asm, but unfortunately, all
current releases of clang use the wrong inline asm constraint codes
for float and double ("w" and "P" instead of "t" and "w",
respectively). this patch adds detection for the bug in configure,
and, for now, just disables the affected asm on broken clang versions.
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in order to take advantage of the fpu in -mfloat-abi=softfp mode, the
__VFP_FP__ (presence of vfp fpu) was checked instead of checking for
__ARM_PCS_VFP (hardfloat EABI variant). however, the latter macro is
the one that's actually specified by the ABI documents rather than
being compiler-specific, and should also be checked in case __VFP_FP__
is not defined on some compilers or some configurations.
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this makes it possible to inline them with LTO, and is the simplest
approach to eliminating the use of .sub files.
this also makes VFP sqrt available for use with the standard EABI
(plain arm rather than armhf subarch) when libc is built with
-mfloat-abi=softfp. the same could have been done for fabs, but when
the argument and return value are in integer registers, moving to VFP
registers and back is almost certainly more costly than a simple
integer operation.
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