Date: Thu, 24 Sep 2015 15:58:12 -0600 From: Ian Lepore <ian@freebsd.org> To: Olavi Kumpulainen <olavi.m.kumpulainen@gmail.com> Cc: freebsd-arm@freebsd.org Subject: Re: gcc-4.2.1 on armv6 and Tag_ABI_HardFP_use Message-ID: <1443131892.1224.312.camel@freebsd.org> In-Reply-To: <64C9409F-7E9D-4CF2-90E1-3834908BC813@gmail.com> References: <64C9409F-7E9D-4CF2-90E1-3834908BC813@gmail.com>
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On Thu, 2015-09-24 at 23:24 +0200, Olavi Kumpulainen wrote:
> Hi everyone,
>
> I have a question regarding armv6 gcc-4.2.1 and Tag_ABI_HardFP_use in the generated ELF objects.
>
> I have a small c-program that looks as;
>
> #include <stdio.h>
>
> double fn(double x, double y)
> {
> return x+y;
> }
>
> int main(int argc, char *argv[])
> {
> printf("%f\n", fn(3.2, 9.7));
> return 0;
> }
>
>
> Compiling this with clang 3.4.1 produces the following results;
>
> $ cc -v
> FreeBSD clang version 3.4.1 (tags/RELEASE_34/dot1-final 208032) 20140512
> Target: armv6--freebsd10.2-gnueabi
> Thread model: posix
> Selected GCC installation:
>
> $ cc -c t.c -march=armv6 -mfloat-abi=soft
> $ readelf -a t.o|grep ABI
> OS/ABI: UNIX - System V
> ABI Version: 0
> Flags: 0x5000000, Version5 EABI
> Tag_ABI_FP_denormal: Needed
> Tag_ABI_FP_exceptions: Needed
> Tag_ABI_FP_number_model: IEEE 754
> Tag_ABI_align8_needed: Yes
> Tag_ABI_align8_preserved: Yes, except leaf SP
>
> $ cc -c t.c -march=armv6 -mfloat-abi=softfp
> $ readelf -a t.o|grep ABI
> OS/ABI: UNIX - System V
> ABI Version: 0
> Flags: 0x5000000, Version5 EABI
> Tag_ABI_FP_denormal: Needed
> Tag_ABI_FP_exceptions: Needed
> Tag_ABI_FP_number_model: IEEE 754
> Tag_ABI_align8_needed: Yes
> Tag_ABI_align8_preserved: Yes, except leaf SP
>
>
> Then, if I build a new world using the following src.conf:
>
> $ cat /etc/src.conf
> WITH_GCC=yes
> WITH_GNUCXX=yes
> WITH_GCC_BOOTSTRAP=yes
> WITHOUT_CLANG=yes
> WITHOUT_CLANG_IS_CC=yes
> WITHOUT_CLANG_BOOTSTRAP=yes
>
> And the usual;
> make TARGET=arm TARGET_ARCH=armv6 buildworld
>
> and;
> make TARGET=arm TARGET_ARCH=armv6 buildenv
>
>
> # cc -v
> Using built-in specs.
> Target: armv6-undermydesk-freebsd
> Configured with: FreeBSD/armv6 system compiler
> Thread model: posix
> gcc version 4.2.1 20070831 patched [FreeBSD]
>
> # cc -c t.c -march=armv6 -mfloat-abi=soft
> # readelf -A t.o |grep ABI
> OS/ABI: UNIX - System V
> ABI Version: 0
> Flags: 0x4000000, Version4 EABI
> Tag_ABI_PCS_wchar_t: 4
> Tag_ABI_FP_denormal: Needed
> Tag_ABI_FP_exceptions: Needed
> Tag_ABI_FP_number_model: IEEE 754
> Tag_ABI_align8_needed: Yes
> Tag_ABI_align8_preserved: Yes, except leaf SP
> Tag_ABI_enum_size: int
> Tag_ABI_optimization_goals: Aggressive Debug
>
> A few more arc-specific tags, but none that strikes me as odd.
>
> But what about -mfloat-abi=softfp?
>
> # cc -c t.c -march=armv6 -mfloat-abi=softfp
> # readelf -a t.o |grep ABI
> OS/ABI: UNIX - System V
> ABI Version: 0
> Flags: 0x4000000, Version4 EABI
> Tag_ABI_PCS_wchar_t: 4
> Tag_ABI_FP_denormal: Needed
> Tag_ABI_FP_exceptions: Needed
> Tag_ABI_FP_number_model: IEEE 754
> Tag_ABI_align8_needed: Yes
> Tag_ABI_align8_preserved: Yes, except leaf SP
> Tag_ABI_enum_size: int
> Tag_ABI_HardFP_use: SP and DP
> Tag_ABI_optimization_goals: Aggressive Debug
>
> Are there any reasons for the appearance of the 'Tag_ABI_HardFP_use: SP and DP˙ here?
>
>
> /O
That "SP and DP" corresponds to a value of 0x03 in the tag, and that is
now listed as a deprecated value which is a synonym for a tag value of
zero (or no tag present at all). The only value these days that has any
meaning for Tag_ABI_HardFP_use is 0x01 which would mean basically "use
only single-precision". I think this old value still appears when using
gcc 4.2 because our gcc tools are so old.
If you use -mfloat-abi=softfp you get hardware floating point (in a form
compatible with libraries and programs that are compiled as
software-only). The performance difference between -mfloat-abi=softfp
and -mfloat-abi=hard is small for most programs, but there is no real
reason to use anything these days except armv6hf which is pure
hardfloat.
-- Ian
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