Re: [PATCH] powerpc/32: Remove one insn in __bswapdi2

On Thu, Aug 11, 2016 at 11:34:37PM +0200, Gabriel Paubert wrote:

On the other hand gcc did at the time a very poor job (quite an
understatement) at bswapdi when compiling for 64 bit processors 
(see the example).

But what do modern compilers generate for bswapdi these days? Do they
still call the library or not?

Nope.

After all, bswapdi on 32 bit processors only takes 6 instructions if the
input and output registers don't overlap.

For this testcase:
===
typedef unsigned long long u64;
u64 bs(u64 x) { return __builtin_bswap64(x); }
===

we get with -m32:
===
bs:
	mr 9,3
	rotlwi 3,4,24
	rlwimi 3,4,8,8,15
	rlwimi 3,4,8,24,31
	rotlwi 4,9,24
	rlwimi 4,9,8,8,15
	rlwimi 4,9,8,24,31
	blr
===

and with -m64:
===
.L.bs:
	srdi 10,3,32
	mr 9,3
	rotlwi 3,3,24
	rotlwi 8,10,24
	rlwimi 3,9,8,8,15
	rlwimi 8,10,8,8,15
	rlwimi 3,9,8,24,31
	rlwimi 8,10,8,24,31
	sldi 3,3,32
	or 3,3,8
	blr
===

Neither as tight as possible, but neither horrible either.


Segher