[LLVMdev] rotate

[reed kotler]

Nice!

Clever compiler..


On 07/28/2012 08:55 PM, Michael Gottesman wrote:
> I can get clang/llvm to emit a rotate instruction on x86-64 when compiling C by just using -Os and the rotate from Hacker's Delight i.e.,
>
> ======
> #include<stdlib.h>
> #include<stdint.h>
>
> uint32_t ror(uint32_t input, size_t rot_bits)
> {
>    return (input>>  rot_bits) | (input<<  ((sizeof(input)<<  3) - rot_bits));
> }
> ======
>
> Then compile with (assuming you are on OS X):
> ======
> ISYSROOT=$(xcodebuild -sdk macosx -version PlatformPath)/Developer/SDKs/MacOSX10.8.sdk
> $(xcrun -find clang) -isysroot $ISYSROOT ror.c -c -S -Os -o -
> ======
>
> yielding an assembly output of:
> ======
> 	.section	__TEXT,__text,regular,pure_instructions
> 	.globl	_rotr
> _rotr:                                  ## @rotr
> 	.cfi_startproc
> ## BB#0:
> 	pushq	%rbp
> Ltmp2:
> 	.cfi_def_cfa_offset 16
> Ltmp3:
> 	.cfi_offset %rbp, -16
> 	movq	%rsp, %rbp
> Ltmp4:
> 	.cfi_def_cfa_register %rbp
> 	movb	%sil, %cl
> 	rorl	%cl, %edi<==== Rotate instruction
> 	movl	%edi, %eax
> 	popq	%rbp
> 	ret
> 	.cfi_endproc
> .subsections_via_symbols
> ======
>
> I hope this helps.
>
> Michael
>
> On Jul 28, 2012, at 8:29 PM, reed kotler<rkotler at mips.com>  wrote:
>
>> in C or C++, how can I get clang/llvm to try and do a "rotate".
>>
>> (want to test this code in the mips16 port)
>>
>> i.e. emit rotr node.
>>
>> tia.
>>
>> reed
>>
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