How to build glibc with HW FPU properly

[VVIsaev]

I tried to build glibc with following configuration:

$ CROSS_COMPILE=arc64-linux-gnu- ASFLAGS='-mfpu=fpud' CFLAGS='-O2 -mfpu=fpud' CXXFLAGS='-O2 -mfpu=fpud' ../configure --target=arc64-linux-gnu --host=arc64-linux-gnu --build=x86_64 --enable-shared --disable-profile --disable-werror --without-gd --prefix=/home/isaev/tmp/glibc-gnuhf
$ make

Then I checked __longjmp() in libc.so.6, see source:

...
        LDR.as r24,   r0, 15

#if defined(__ARCV3__)
        LDR.as r25,   r0, 16
        LDR.as r26,   r0, 17
#endif

#if defined (__ARC_FLOAT_ABI_HARD__)
        FLDR.as f16,  r0, 18
...
        FLDR.as f31,  r0, 33
#endif

        MOVR.f r0, r1
        j.d    [blink]
...

But in built libc I do't see such code:

0000000000035608 <__longjmp>:
...
   35644:       100f 0e58               ldl.as  r24,[r0,15]
   35648:       1010 0e59               ldl.as  r25,[r0,16]
   3564c:       1011 0e5a               ldl.as  r26,[r0,17]
   35650:       580a 8040               movl.f  r0,r1
   35654:       2021 07c0               j.d     [blink]
...

So I don't see fp related code in __longjmp.

[abrodkin]

Somehow adding libc_cv_cc_submachine="-mfpu=fpud" to your configure command-line helps. Likely due to https://sourceware.org/bugzilla/show_bug.cgi?id=10118 & a fix for it https://sourceware.org/bugzilla/attachment.cgi?id=3927&action=diff:

000000000003570c <__longjmp>:
   3570c:»      1000 0e5f           »   ldl.as» blink,[r0]
   35710:»      1001 0e5c           »   ldl.as» sp,[r0,1]
   35714:»      1002 0e5b           »   ldl.as» fp,[r0,2]
   35718:»      1003 0e5e           »   ldl.as» r30,[r0,3]
   3571c:»      1004 0e4d           »   ldl.as» r13,[r0,4]
   35720:»      1005 0e4e           »   ldl.as» r14,[r0,5]
   35724:»      1006 0e4f           »   ldl.as» r15,[r0,6]
   35728:»      1007 0e50           »   ldl.as» r16,[r0,7]
   3572c:»      1008 0e51           »   ldl.as» r17,[r0,8]
   35730:»      1009 0e52           »   ldl.as» r18,[r0,9]
   35734:»      100a 0e53           »   ldl.as» r19,[r0,10]
   35738:»      100b 0e54           »   ldl.as» r20,[r0,11]
   3573c:»      100c 0e55           »   ldl.as» r21,[r0,12]
   35740:»      100d 0e56           »   ldl.as» r22,[r0,13]
   35744:»      100e 0e57           »   ldl.as» r23,[r0,14]
   35748:»      100f 0e58           »   ldl.as» r24,[r0,15]
   3574c:»      1010 0e59           »   ldl.as» r25,[r0,16]
   35750:»      1011 0e5a           »   ldl.as» r26,[r0,17]
   35754:»      e012 041a           »   fld64.as»       f16,[r0,18]
   35758:»      e013 045a           »   fld64.as»       f17,[r0,19]
   3575c:»      e014 049a           »   fld64.as»       f18,[r0,20]
   35760:»      e015 04da           »   fld64.as»       f19,[r0,21]
   35764:»      e016 051a           »   fld64.as»       f20,[r0,22]
   35768:»      e017 055a           »   fld64.as»       f21,[r0,23]
   3576c:»      e018 059a           »   fld64.as»       f22,[r0,24]
   35770:»      e019 05da           »   fld64.as»       f23,[r0,25]
   35774:»      e01a 061a           »   fld64.as»       f24,[r0,26]
   35778:»      e01b 065a           »   fld64.as»       f25,[r0,27]
   3577c:»      e01c 069a           »   fld64.as»       f26,[r0,28]
   35780:»      e01d 06da           »   fld64.as»       f27,[r0,29]
   35784:»      e01e 071a           »   fld64.as»       f28,[r0,30]
   35788:»      e01f 075a           »   fld64.as»       f29,[r0,31]
   3578c:»      e020 079a           »   fld64.as»       f30,[r0,32]
   35790:»      e021 07da           »   fld64.as»       f31,[r0,33]
   35794:»      580a 8040           »   movl.f» r0,r1
   35798:»      2021 07c0           »   j.d»    [blink]
   3579c:»      20ca 0061           »   mov.eq» r0,0x1

And now we need to figure out how to do that consistently. My guess is configuring GCC with --with-fpu=fpud (like we do in Buildroot when enabled BR2_GCC_TARGET_FPU, see https://github.com/foss-for-synopsys-dwc-arc-processors/buildroot/blob/arc64/arch/Config.in.arc#L111) should be good enough as then regardless of ASFLAGS GCC will have __ARC_FLOAT_ABI_HARD__ defined by default.