[PATCH v4 2/9] ARM: assembler: introduce bl_r macro

[Ard Biesheuvel]

Add a bl_r macro that abstract the difference between the ways indirect
calls are performed on older and newer ARM architecture revisions.

The main difference is to prefer blx instructions over explicit LR
assignments when possible, as these tend to confuse the prediction logic
in out-of-order cores when speculating across a function return.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
Reviewed-by: Arnd Bergmann <arnd at arndb.de>
Acked-by: Linus Walleij <linus.walleij at linaro.org>
Tested-by: Keith Packard <keithpac at amazon.com>
---
 arch/arm/include/asm/assembler.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
index 7d23d4bb2168..870bfaea4318 100644
--- a/arch/arm/include/asm/assembler.h
+++ b/arch/arm/include/asm/assembler.h
@@ -624,4 +624,19 @@ THUMB(	orr	\reg , \reg , #PSR_T_BIT	)
 	.endif
 	.endm
 
+	/*
+	 * bl_r - branch and link to register
+	 *
+	 * @dst: target to branch to
+	 * @c: conditional opcode suffix
+	 */
+	.macro		bl_r, dst:req, c
+	.if		__LINUX_ARM_ARCH__ < 6
+	mov\c		lr, pc
+	mov\c		pc, \dst
+	.else
+	blx\c		\dst
+	.endif
+	.endm
+
 #endif /* __ASM_ASSEMBLER_H__ */
-- 
2.30.2