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From: Vineet Gupta <vgupta@sourceware.org> To: glibc-cvs@sourceware.org Subject: [glibc] ARC: fp: (micro)optimize FPU_STATUS read by eliding FWE bit clearing Date: Wed, 21 Jul 2021 20:15:26 +0000 (GMT) [thread overview] Message-ID: <20210721201526.6AC183857C71@sourceware.org> (raw) https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=31aefa93f3e9a49b7a493d410acb70108e176d61 commit 31aefa93f3e9a49b7a493d410acb70108e176d61 Author: Vineet Gupta <vgupta@synopsys.com> Date: Fri Jul 9 14:56:18 2021 -0700 ARC: fp: (micro)optimize FPU_STATUS read by eliding FWE bit clearing Any FPU_STATUS write needs setting the FWE bit (31) whcih just provides a "control signal" to enable explicit write (vs. the side-effect of FPU instructions). However this bit is RAZ and write-only, thus effectively never stored in FPU_STATUS register. Thus when reading the register there is no need to clear it. This shaves off a BCLR instruction from the fe*exceptino family of functions and while no big deal still makes sense to do. This came up when debugging a race in math/test-fenv-tls [1] [1]: https://github.com/foss-for-synopsys-dwc-arc-processors/linux/issues/54 Signed-off-by: Vineet Gupta <vgupta@synopsys.com> Diff: --- sysdeps/arc/fpu_control.h | 15 +++++++-------- 1 file changed, 7 insertions(+), 8 deletions(-) diff --git a/sysdeps/arc/fpu_control.h b/sysdeps/arc/fpu_control.h index c7d101e783..ae4348321c 100644 --- a/sysdeps/arc/fpu_control.h +++ b/sysdeps/arc/fpu_control.h @@ -81,21 +81,20 @@ typedef unsigned int fpu_control_t; # define _FPU_SETCW(cw) __asm__ volatile ("sr %0, [0x300]" : : "r" (cw)) /* Macros for accessing the hardware status word. - FWE bit is special as it controls if actual status bits could be wrritten - explicitly (other than FPU instructions). We handle it here to keep the - callers agnostic of it: - - clear it out when reporting status bits - - always set it when changing status bits. */ + Writing to FPU_STATUS requires a "control" bit FWE to be able to set the + exception flags directly (as opposed to side-effects of FP instructions). + That is done in the macro here to keeps callers agnostic of this detail. + And given FWE is write-only and RAZ, no need to "clear" it in _FPU_GETS + macro. */ # define _FPU_GETS(cw) \ __asm__ volatile ("lr %0, [0x301] \r\n" \ - "bclr %0, %0, 31 \r\n" \ : "=r" (cw)) # define _FPU_SETS(cw) \ do { \ - unsigned int __tmp = 0x80000000 | (cw); \ + unsigned int __fwe = 0x80000000 | (cw); \ __asm__ volatile ("sr %0, [0x301] \r\n" \ - : : "r" (__tmp)); \ + : : "r" (__fwe)); \ } while (0) /* Default control word set at startup. */
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