[cfe-dev] disabling loop idiom recognizer in clang

[Damjan Marion]

On Apr 29, 2011, at 12:59 AM, jahanian wrote:

> 
> On Apr 28, 2011, at 3:49 PM, Damjan Marion wrote:
> 
>> 
>> On Apr 29, 2011, at 12:40 AM, Andrew Fish wrote:
>> 
>>> On Apr 28, 2011, at 3:17 PM, Damjan Marion wrote:
>>> 
>>>> 
>>>> On Apr 29, 2011, at 12:15 AM, Chris Lattner wrote:
>>>> 
>>>>> 
>>>>> On Apr 28, 2011, at 3:09 PM, Damjan Marion wrote:
>>>>> 
>>>>>> 
>>>>>> Hi,
>>>>>> 
>>>>>> is there a way to disable loop idiom recognizer in clang?
>>>>>> 
>>>>>> I have a problem with compiling early stage code for ARM architecture which should not be linked to external libraries but clang replaces loop with memset call in the code.
>>> 
>>> If it is just the loop you care about make the pointer volatile. 
>>> 
>>>>> 
>>>>> Yep, build with -fno-builtin or -ffreestanding.
>>>> 
>>>> It doesn't work. According to gcc docs (which clang probably follows): 
>>>> "GCC requires the freestanding environment provide memcpy, memmove, memset and memcmp."
>>> 
>>> 
>>> It works great, you just need your own copies of the functions.  That is what we did for ARM EFI firmware, for gcc, clang, and RVCT.  https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2/ArmPkg/Library/CompilerIntrinsicsLib/Arm/. Sorry no makefiles as they get auto-generated from ../ CompilerIntrinsicsLib.inf. No floating point, but no external libraries means well no external libraries! 
>> 
>> In my case writing my own functions is slightly overkill for just avoiding clang to convert "for (i = 0; i <17; i++) c [i] = 0;" to memset. Must be a way to tell him not to mess with my loop :)
> 
> Can you write it in a way that memset  conversion logic does not recognize it; such as i =0; for (; i <17; i++) c [i] = 0;

It is too smart :)

# arm-none-eabi-objdump -S inflate-tramp.o | grep -B 2 -A 2 memset
     b64:	858d0030 	strhi	r0, [sp, #48]	; 0x30
     b68:	e24b0064 	sub	r0, fp, #100	; 0x64
     b6c:	ebfffffe 	bl	0 <memset>
	i=0; for (; i < BMAX+1; i++) c [i] = 0;