[PATCH v2] ARM: Use udiv/sdiv for __aeabi_{u}idiv library functions

On Tue, 12 Nov 2013, Russell King - ARM Linux wrote:

On Tue, Nov 12, 2013 at 09:01:16AM -0500, Nicolas Pitre wrote:

quoted

What about this patch which I think is currently your best option.  Note 
it would need to use the facilities from asm/opcodes.h to make it endian 
agnostic.

diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
index 6a1b8a81b1..379cffe4ab 100644
--- a/arch/arm/kernel/setup.c
+++ b/arch/arm/kernel/setup.c

@@ -383,6 +383,34 @@ static void __init cpuid_init_hwcaps(void)
 		elf_hwcap |= HWCAP_IDIVT;
 	}
 
+	/*
+	 * Patch our division routines with the corresponding opcode
+	 * if the hardware supports it.
+	 */
+	if (IS_ENABLED(CONFIG_THUMB2_KERNEL) && (elf_hwcap & HWCAP_IDIVT)) {
+		extern char __aeabi_uidiv, __aeabi_idiv;
+		u16 *uidiv = (u16 *)&__aeabi_uidiv;
+		u16 *idiv = (u16 *)&__aeabi_idiv;
+
+		uidiv[0] = 0xfbb0;  /* udiv r0, r0, r1 */
+		uidiv[1] = 0xf0f1;
+		uidiv[2] = 0x4770;  /* bx lr */
+
+		idiv[0] = 0xfb90;  /* sdiv r0, r0, r1 */
+		idiv[1] = 0xf0f1;  
+		idiv[2] = 0x4770;  /* bx lr */
+	} else if (!IS_ENABLED(CONFIG_THUMB2_KERNEL) && (elf_hwcap & HWCAP_IDIVA)) {
+		extern char __aeabi_uidiv, __aeabi_idiv;
+		u32 *uidiv = (u32 *)&__aeabi_uidiv;
+		u32 *idiv = (u32 *)&__aeabi_idiv;
+
+		uidiv[0] = 0xe730f110;  /* udiv r0, r0, r1 */
+		uidiv[1] = 0xe12fff1e;  /* bx lr */
+
+		idiv[0] = 0xe710f110;  /* sdiv r0, r0, r1 */
+		idiv[1] = 0xe12fff1e;  /* bx lr */
+	}

What about endianness, and what if XIP is enabled?

Just as I said above the diff: this needs refined.

Obviously XIP can't use this and doesn't need it either as a XIP kernel 
should be optimized for the very platform it will run onto i.e. gcc 
should already emit those div opcodes inline if appropriate.


Nicolas