On Tue, Oct 19, 2021 at 11:14:47AM +0100, Will Deacon wrote:
> On Mon, Oct 18, 2021 at 08:08:20PM +0100, Mark Brown wrote:
> > SYM_FUNC_START(sve_flush_live)
> > - cbz x0, 1f // A VQ-1 of 0 is 128 bits so no extra Z state
> > + cbz x1, 1f // A VQ-1 of 0 is 128 bits so no extra Z state
> > sve_flush_z
> > -1: sve_flush_p_ffr
> > +1: cbz x0, 2f
> > + sve_flush_p
> > +2: sve_flush_ffr
> > ret
> > @@ -962,7 +962,7 @@ void do_sve_acc(unsigned int esr, struct pt_regs *regs)
> > unsigned long vq_minus_one =
> > sve_vq_from_vl(current->thread.sve_vl) - 1;
> > sve_set_vq(vq_minus_one);
> > - sve_flush_live(vq_minus_one);
> > + sve_flush_live(true, vq_minus_one);
>
> What does the pcs say about passing bools in registers? Can we guarantee
> that false is a 64-bit zero?
Per usual rules, bits [63:8] can be arbitrary -- AAPCS64 leaves it to the callee
to extend values, with the upper bits being arbitrary, and it maps _Bool/bool
to unsigned char, which covers bits [7:0].
So a bool false in a register is not guaranteed to be a 64-bit zero. But
since it *is* guarnateed to be either 0 or 1, we can use TBZ/TBNZ
instead of CBZ/CBNZ. Either that, or extend it to a wider type in the
function prototype.
The test below shows clang and GCC both agree with that (though this old
GCC seems to do unnecessary zero extension as a caller):
| [mark@gravadlaks:~]% cat bool.c
| #include <stdbool.h>
|
| void callee_bool(bool b);
|
| void callee_unsigned_int(unsigned int i);
|
| void caller_unsigned_long(unsigned long l)
| {
| unsigned long tmp = l & 0xffffffff;
|
| if (tmp)
| callee_unsigned_int(tmp);
| else
| callee_bool(tmp);
| }
|
| unsigned long bool_to_unsigned_long(bool b)
| {
| return b;
| }
| [mark@gravadlaks:~]% gcc --version
| gcc (Debian 8.3.0-6) 8.3.0
| Copyright (C) 2018 Free Software Foundation, Inc.
| This is free software; see the source for copying conditions. There is NO
| warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
|
| [mark@gravadlaks:~]% gcc -c bool.c -O3
| [mark@gravadlaks:~]% objdump -d bool.o
|
| bool.o: file format elf64-littleaarch64
|
|
| Disassembly of section .text:
|
| 0000000000000000 <caller_unsigned_long>:
| 0: 34000040 cbz w0, 8 <caller_unsigned_long+0x8>
| 4: 14000000 b 0 <callee_unsigned_int>
| 8: 52800000 mov w0, #0x0 // #0
| c: 14000000 b 0 <callee_bool>
|
| 0000000000000010 <bool_to_unsigned_long>:
| 10: 92401c00 and x0, x0, #0xff
| 14: d65f03c0 ret
| [mark@gravadlaks:~]% clang --version
| clang version 7.0.1-8+deb10u2 (tags/RELEASE_701/final)
| Target: aarch64-unknown-linux-gnu
| Thread model: posix
| InstalledDir: /usr/bin
| [mark@gravadlaks:~]% clang -c bool.c -O3
| [mark@gravadlaks:~]% objdump -d bool.o
|
| bool.o: file format elf64-littleaarch64
|
|
| Disassembly of section .text:
|
| 0000000000000000 <caller_unsigned_long>:
| 0: 34000040 cbz w0, 8 <caller_unsigned_long+0x8>
| 4: 14000000 b 0 <callee_unsigned_int>
| 8: 14000000 b 0 <callee_bool>
|
| 000000000000000c <bool_to_unsigned_long>:
| c: 92400000 and x0, x0, #0x1
| 10: d65f03c0 ret
Thanks,
Mark.
