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.