Comments on: Beware the builtins

By: stevenb

stevenb — Sun, 07 Feb 2010 15:47:57 +0000

The 64 bit variant compiles to fairly good code with "GCC 4.5.0 [trunk revision 156572]". At -O2 the compiler produces the following for ARM7:

__bswapdi2:
	mov	r3, r0
	eor	r2, r0, r0, ror #16
	eor	r0, r1, r1, ror #16
	mov	r2, r2, lsr #8
	mov	r0, r0, lsr #8
	bic	r2, r2, #65280
	bic	r0, r0, #65280
	eor	r0, r0, r1, ror #8
	eor	r1, r2, r3, ror #8
	bx	lr

By: stevenb

stevenb — Fri, 05 Feb 2010 11:22:46 +0000

There was a patch to GCC last week that should make bswap32 should work now (although I have not checked):

* config/arm/arm.md (bswapsi2): Add support for bswapsi2.
(arm_rev): New.
(arm_legacy_rev): Likewise.
(thumb_legacy_rev): Likewise.

By: Mans

Mans — Fri, 29 Jan 2010 18:22:30 +0000

Byte-reversing load/store is, in my view, a different class of instruction. I would not say a machine “has byte-reversal” unless it can perform that operation on typical operands, which in the case of PPC means registers.

I am pleased to see, however, that __builtin_bswap32 applied to a memory location does in fact make use of lwbrx.

By: Fredrik Andersson

Fredrik Andersson — Fri, 29 Jan 2010 17:59:52 +0000

The powerpc actually do have bytereversal, although it can only operate on memory, lwbrx for example (thus useful when you load data)

By: Reimar

Reimar — Sat, 23 Jan 2010 15:57:54 +0000

Every time the same mess:
mov r5, #0
and r7, r0, r5
gcc desperately needs to learn not to do multiplies, ands and adds with 0.
But I guess the truth is nobody cares about the operand size > native size case.

By: Nils

Nils — Thu, 14 Jan 2010 17:56:14 +0000

I like your two instruction __builtin_ctz version. Haven’t thought about using bit-reverse for this.. Nice!

By: Daniel Jacobowitz

Daniel Jacobowitz — Thu, 14 Jan 2010 15:06:35 +0000

FYI, ARM __builtin_ctz is done (http://gcc.gnu.org/ml/gcc-patches/2009-07/msg00467.html). bswap32 was posted at least twice, too (latest http://gcc.gnu.org/ml/gcc-patches/2009-06/msg01100.html). I've poked the maintainers about that one. As for bswap64, well... that's just gross.