Re: [boost] Suggest: detail functions for full built-in arithmetic

From: Michael Marcin (mmarcin_at_[hidden])
Date: 2007-09-10 00:26:03

Phil Endecott wrote:
> Guillaume Melquiond wrote:
>> Le samedi 08 septembre 2007 ? 23:10 -0400, Daryle Walker a ?crit :
>>> Maybe we should add a header to our detail (and/or pending) folder 
>>> encapsulating the full versions of built-in integer arithmetic 
>>> operations, that most(?) processors have, but neither C nor C++ let us 
>>> access.  I'm talking about functions like:
>>>
>>> std::pair<unsigned, bool>  full_add( unsigned a1, unsigned a2 );
>>>
>>> The processor has a "carry" flag whenever an integer addition overflows, 
>>> but we can't access it (without assembly-language hacks).
>> Is it really worth it? Nowadays, compilers are smart enough. They
>> generate better code when details are not obfuscated by hand-crafted
>> assembly code. I tested the following code with GCC:
>>
>>         #include <algorithm>
>>         
>>         std::pair<unsigned, bool> full_add(unsigned a, unsigned b)
>>         { return std::make_pair(a + b, a + b < a); }
>>         
>>         bool no_overflow(unsigned a, unsigned b)
>>         { return !full_add(a, b).second; }
> 
> Here's what I get on ARM:
> 
> _Z8full_addjj:
> 	adds	r2, r1, r2
> 	movcc	r1, #0
> 	movcs	r1, #1
> 	str	r2, [r0, #0]
> 	strb	r1, [r0, #4]
> 	mov	pc, lr
> 
> _Z11no_overflowjj:
> 	cmn	r0, r1
> 	movcs	r0, #0
> 	movcc	r0, #1
> 	mov	pc, lr
> 
> 

RVCT 2.2 with --arm and -O2

Gives:

_Z8full_addjj PROC
         PUSH     {r2,r3,lr}
         ADD      r2,r1,r2
         CMP      r2,r1
         MOVCS    r1,#0
         MOVCC    r1,#1
         STR      r1,[sp,#0]
         STR      r2,[sp,#4]
         STR      r2,[r0,#0]
         LDRB     r1,[sp,#0]
         STRB     r1,[r0,#4]
         POP      {r3,r12,pc}
         ENDP

_Z11no_overflowjj PROC
         PUSH     {r0-r3,lr}
         MOV      r2,r1
         MOV      r1,r0
         ADD      r0,sp,#8
         BL       _Z8full_addjj
         LDR      r0,[sp,#8]
         LDR      r1,[sp,#0xc]
         STM      sp,{r0,r1}
         LDRB     r0,[sp,#4]
         RSBS     r0,r0,#1
         MOVCC    r0,#0
         POP      {r1-r3,r12,pc}
         ENDP

Seems just a tad worse than what you got :)
What GCC did you use?

Thanks,

Michael Marcin