[llvm-commits] [llvm] r110799 - in /llvm/trunk: lib/Target/ARM/ARMISelLowering.cpp lib/Target/ARM/ARMISelLowering.h test/CodeGen/ARM/select.ll

[Bill Wendling]

On Aug 19, 2010, at 3:58 PM, John Tytgat wrote:

> In message <20100811084316.8A1092A6C12C at llvm.org> you wrote:
> 
>> Author: void
>> Date: Wed Aug 11 03:43:16 2010
>> New Revision: 110799
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=110799&view=rev
>> Log:
>> Consider this code snippet:
>> 
>> float t1(int argc) {
>>  return (argc == 1123) ? 1.234f : 2.38213f;
>> }
>> [...]
>> 
>> Now we generate this, which looks optimal to me:
>> 
>> _t1:
>>  movw   r1, #1123
>>  movs   r2, #0
>>  cmp    r0, r1
>>  adr    r0, #LCPI0_0
>>  it     eq
>>  moveq  r2, #4
>>  ldr    r0, [r0, r2]
>>  bx     lr
>>  .align  2
>> LCPI0_0:
>>  .long   1075344593  @ float 2.382130e+00
>>  .long   1067316150  @ float 1.234000e+00
> 
> I don't know enough about Thumb to be sure (much more familiar with ARM)
> but can't this be made more compact/faster using SUB instead of CMP
> eliminating the MOV Rx, #0 ?
> 
> _t1:
>  movw   r1, #1123
>  subs   r2, r0, r1
>  adr    r0, #LCPI0_0
>  it     ne
>  movne  r2, #4
>  ldr    r0, [r0, r2]
>  bx     lr
>  .align  2
> LCPI0_0:
>  .long   1067316150  @ float 1.234000e+00
>  .long   1075344593  @ float 2.382130e+00
> 
Steve Canon pointed out that this is optimal:

_t1:
	movw 	r1, 	#1123
	adr		r2,	#LCPI0_0
	cmp		r0,	r1
	it		eq
	addeq	r2,	r2,	#4
	ldr		r0,	[r2]
	bx		lr

Essentially, folding the offset increment into the 'it' block.

-bw