On Sun, 25 Jul 2021, Zoltán Kócsi wrote:
I try to write a one-liner inline function to create a double form
a 64-bit integer, not converting it to a double but the integer
containing the bit pattern for the double (type spoofing).
The compiler is arm-eabi-gcc 8.2.0.
The target is a Cortex-A9, with NEON.
According to the info page the assembler constraint "w" denotes an FPU
double register, d0 - d31.
The code is the following:
double spoof( uint64_t x )
{
double r;
asm volatile
(
" vmov.64 %[d],%Q[i],%R[i] \n"
Isn't it supposed to be %P[d] for a double?
(the documentation is very lacking...)
: [d] "=w" (r)
: [i] "q" (x)
);
return r;
}
The command line:
arm-eabi-gcc -O0 -c -mcpu=cortex-a9 -mfloat-abi=hard -mfpu=neon-vfpv4 \
test.c
It compiles and the generated object code is this:
00000000 <spoof>:
0: e52db004 push {fp} ; (str fp, [sp, #-4]!)
4: e28db000 add fp, sp, #0
8: e24dd014 sub sp, sp, #20
c: e14b01f4 strd r0, [fp, #-20] ; 0xffffffec
10: e14b21d4 ldrd r2, [fp, #-20] ; 0xffffffec
14: ec432b30 vmov d16, r2, r3
18: ed4b0b03 vstr d16, [fp, #-12]
1c: e14b20dc ldrd r2, [fp, #-12]
20: ec432b30 vmov d16, r2, r3
24: eeb00b60 vmov.f64 d0, d16
28: e28bd000 add sp, fp, #0
2c: e49db004 pop {fp} ; (ldr fp, [sp], #4)
30: e12fff1e bx lr
which is not really efficient, but works.
However, if I specify -O1, -O2 or -Os then the compilation fails
because assembler complains. This is the assembly the compiler
generated, (comments and irrelevant stuff removed):
spoof:
vmov.64 s0,r0,r1
bx lr
where the problem is that 's0' is a single-precision float register and
it should be 'd0' instead.
Either I'm seriously missing something, in which case I would be most
obliged if someone sent me to the right direction; or it is a compiler
or documentation bug.
Thanks,
Zoltan
--
Marc Glisse
