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[COMMITTED] RISC-V: Execute a PAUSE hint in spin loops

Message ID 20240624193902.22596-1-dilfridge@gentoo.org
State New
Headers show
Series [COMMITTED] RISC-V: Execute a PAUSE hint in spin loops | expand

Commit Message

Andreas K. Huettel June 24, 2024, 7:38 p.m. UTC 
From: Christoph Müllner <christoph.muellner@vrull.eu>

The atomic_spin_nop() macro can be used to run arch-specific
code in the body of a spin loop to potentially improve efficiency.
RISC-V's Zihintpause extension includes a PAUSE instruction for
this use-case, which is encoded as a HINT, which means that it
behaves like a NOP on systems that don't implement Zihintpause.

Binutils supports Zihintpause since 2.36, so this patch uses
the ".insn" directive to keep the code compatible with older
toolchains.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
---
 sysdeps/unix/sysv/linux/riscv/atomic-machine.h | 3 +++
 1 file changed, 3 insertions(+)
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Patch

diff --git a/sysdeps/unix/sysv/linux/riscv/atomic-machine.h b/sysdeps/unix/sysv/linux/riscv/atomic-machine.h
index c1c9d949a0..90283d9746 100644
--- a/sysdeps/unix/sysv/linux/riscv/atomic-machine.h
+++ b/sysdeps/unix/sysv/linux/riscv/atomic-machine.h
@@ -178,4 +178,7 @@ 
 # error "ISAs that do not subsume the A extension are not supported"
 #endif /* !__riscv_atomic */
 
+/* Execute a PAUSE hint when spinning.  */
+#define atomic_spin_nop() __asm(".insn i 0x0f, 0, x0, x0, 0x010")
+
 #endif /* bits/atomic.h */