linux-kernel - [PATCH] MIPS: dec: Avoid la pseudo-instruction in delay slots

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Date:   Fri, 2 Sep 2016 15:19:01 +0100
From:   Paul Burton <paul.burton@...tec.com>
To:     <linux-mips@...ux-mips.org>
CC:     Paul Burton <paul.burton@...tec.com>,
        "Maciej W. Rozycki" <macro@...ux-mips.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        <linux-kernel@...r.kernel.org>
Subject: [PATCH] MIPS: dec: Avoid la pseudo-instruction in delay slots

When expanding the la or dla pseudo-instruction in a delay slot the GNU
assembler will complain should the pseudo-instruction expand to multiple
actual instructions, since only the first of them will be in the delay
slot leading to the pseudo-instruction being only partially executed if
the branch is taken. Use of PTR_LA in the dec int-handler.S leads to
such warnings:

  arch/mips/dec/int-handler.S: Assembler messages:
  arch/mips/dec/int-handler.S:149: Warning: macro instruction expanded into multiple instructions in a branch delay slot
  arch/mips/dec/int-handler.S:198: Warning: macro instruction expanded into multiple instructions in a branch delay slot

Avoid this by placing nops in the delay slots of the affected branches,
leading to the PTR_LA macros being placed after the branches & their
delay slots. Although the nop isn't strictly needed, it's an
insignificant cost & satisfies the assembler easily with more
readable code than the possible alternative of manually expanding the
la/dla pseudo-instructions & placing the appropriate first instruction
into the delay slots.

Signed-off-by: Paul Burton <paul.burton@...tec.com>
---

 arch/mips/dec/int-handler.S | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/mips/dec/int-handler.S b/arch/mips/dec/int-handler.S
index d7b9918..54ddca1 100644
--- a/arch/mips/dec/int-handler.S
+++ b/arch/mips/dec/int-handler.S
@@ -137,16 +137,16 @@
 		and	t0,t1			# isolate allowed ones

 		beqz	t0,spurious
-
 #ifdef CONFIG_32BIT
 		 and	t2,t0
 		bnez	t2,fpu			# handle FPU immediately
 #endif
+		 nop

 		/*
 		 * Find irq with highest priority
 		 */
-		 PTR_LA	t1,cpu_mask_nr_tbl
+		PTR_LA	t1,cpu_mask_nr_tbl
 1:		lw	t2,(t1)
 		nop
 		and	t2,t0
@@ -191,11 +191,12 @@
 1:		and	t0,t1			# mask out allowed ones

 		beqz	t0,spurious
+		 nop

 		/*
 		 * Find irq with highest priority
 		 */
-		 PTR_LA	t1,asic_mask_nr_tbl
+		PTR_LA	t1,asic_mask_nr_tbl
 2:		lw	t2,(t1)
 		nop
 		and	t2,t0
-- 
2.9.3