Will,


On 12/19/2016 02:06 AM, Will Deacon wrote:

On Fri, Dec 16, 2016 at 03:17:51PM -0800, Joshua Clayton wrote:

quoted

Implement faster bitrev8x4() for arm, arm64 and mips, all the platforms
with CONFIG_HAVE_ARCH_BITREVERSE.
ARM platforms just need a byteswap added to the existing __arch_bitrev32()
Amusingly, the mips implementation is exactly the opposite, requiring
removal of the byteswap from its __arch_bitrev32()

Signed-off-by: Joshua Clayton <redacted>
---
 arch/arm/include/asm/bitrev.h   | 6 ++++++
 arch/arm64/include/asm/bitrev.h | 6 ++++++
 arch/mips/include/asm/bitrev.h  | 6 ++++++
 include/linux/bitrev.h          | 1 +
 4 files changed, 19 insertions(+)

diff --git a/arch/arm/include/asm/bitrev.h b/arch/arm/include/asm/bitrev.h
index ec291c3..9482f78 100644
--- a/arch/arm/include/asm/bitrev.h
+++ b/arch/arm/include/asm/bitrev.h

@@ -17,4 +17,10 @@ static __always_inline __attribute_const__ u8 __arch_bitrev8(u8 x)
 	return __arch_bitrev32((u32)x) >> 24;
 }
 
+static __always_inline __attribute_const__ u32 __arch_bitrev8x4(u32 x)
+{
+	__asm__ ("rbit %0, %1; rev %0, %0" : "=r" (x) : "r" (x));
+	return x;
+}
+
 #endif

diff --git a/arch/arm64/include/asm/bitrev.h b/arch/arm64/include/asm/bitrev.h
index a5a0c36..1801078 100644
--- a/arch/arm64/include/asm/bitrev.h
+++ b/arch/arm64/include/asm/bitrev.h

@@ -16,4 +16,10 @@ static __always_inline __attribute_const__ u8 __arch_bitrev8(u8 x)
 	return __arch_bitrev32((u32)x) >> 24;
 }
 
+static __always_inline __attribute_const__ u32 __arch_bitrev8x4(u32 x)
+{
+	__asm__ ("rbit %0, %1; rev %0, %0" : "=r" (x) : "r" (x));

This is broken -- you're operating on 64-bit registers. I only noticed
because if you write:

Ugh. mea culpa. I squinted at the AARCH64 asm and erroneously
believed it to be the same as arm.

  swab32(bitrev32(x))

then GCC generates:

  rbit	w0, w0
  rev	w0, w0

so perhaps we should just implement the asm-generic version like that
and not bother with the arch-specific stuff?

Will

You are so right.
That is exactly what I will do.

Thanks,

Joshua
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