[LLVMdev] Optimization feasibility

[Evan Cheng]

On Dec 25, 2007, at 9:07 AM, Arnold Schwaighofer wrote:

> On 25 Dec 2007, at 03:29, Gordon Henriksen wrote:
>
>> Hi Jo,
>>
>> On 2007-12-24, at 14:43, Joachim Durchholz wrote:
>>
>>> I'm in a very preliminary phase of a language project which requires
>>> some specific optimizations to be reasonably efficient.
>>>
>>> LLVM already looks very good; I'd just like to know whether I can
>>> push these optimizations through LLVM to the JIT phase (which, as
>>> far as I understand the docs, is a pretty powerful part of LLVM).
>>
>> Cool.
>>
>>> The optimizations that I need to get to work are:
>>>
>>> * Tail call elimination.
>
>> It also supports emitting tail calls on
>> x86, but its support is somewhat weak. This is partially mandated by
>> calling conventions, but those implementing functional languages  
>> might
>> be disappointed. Check the llvmdev archives for details.
>>
> Hi Joachim,
> I am  the person to blame for tail call support and its deficiencies
> on x86.

Hi Arnold,

Speaking of tail call support... Do you have any plan to enhance it in  
the near future?

Thanks,

Evan

>
>
> The current constraints for tail calls on x86 are:
>
> Max 2 registers are used for argument passing (inreg). Tail call
> optimization is performed
> provided:
> //                * option tailcallopt is enabled
> //                * caller/callee are fastcc
> //                * elf/pic is disabled (this should be the case on
> mac os x?) OR
> //                * elf/pic enabled + callee is in the same module as
> caller + callee has
> //                  visibility protected or hidden
>
>
> an (pointless) example would be:
>
>
> <<---tailcall.ll --->>
> @.str = internal constant [12 x i8] c"result: %d\0A\00"		; <[12 x i8]
> *> [#uses=1]
>
> define fastcc i32 @tailcallee(i32 %a1, i32 %a2, i32 %a3, i32 %a4) {
> entry:
>         ret i32 %a3
> }
>
> define fastcc i32 @tailcaller(i32 %in1, i32 %in2) {
> entry:
>         %tmp11 = tail call fastcc i32 @tailcallee( i32 %in1, i32 %
> in2, i32 %in1,
>  i32 %in2 )             ; <i32> [#uses=1]
>         ret i32 %tmp11
> }
>
>
>
> define i32 @main(i32 %argc, i8** %argv) {
> entry:
> 	%argc_addr = alloca i32		; <i32*> [#uses=1]
> 	%argv_addr = alloca i8**		; <i8***> [#uses=1]
> 	%retval = alloca i32, align 4		; <i32*> [#uses=2]
> 	%tmp = alloca i32, align 4		; <i32*> [#uses=2]
> 	%res = alloca i32, align 4		; <i32*> [#uses=2]
> 	"alloca point" = bitcast i32 0 to i32		; <i32> [#uses=0]
> 	store i32 %argc, i32* %argc_addr
> 	store i8** %argv, i8*** %argv_addr
> 	%tmp1 = call fastcc i32 @tailcaller( i32 1, i32 2)		; <i32> [#uses=1]
> 	store i32 %tmp1, i32* %res
> 	%tmp2 = getelementptr [12 x i8]* @.str, i32 0, i32 0		; <i8*>  
> [#uses=1]
> 	%tmp3 = load i32* %res		; <i32> [#uses=1]
> 	%tmp4 = call i32 (i8*, ...)* @printf( i8* %tmp2, i32 %tmp3 )		;
> <i32> [#uses=0]
> 	store i32 0, i32* %tmp
> 	%tmp5 = load i32* %tmp		; <i32> [#uses=1]
> 	store i32 %tmp5, i32* %retval
> 	br label %return
>
> return:		; preds = %entry
> 	%retval6 = load i32* %retval		; <i32> [#uses=1]
> 	ret i32 %retval6
> }
>
> declare i32 @printf(i8*, ...)
> <<---tailcall.ll --->>
>
> x86Shell:>  llvm-as < tailcall.ll | llc  -tailcallopt | gcc -x
> assembler -
> x86Shell:> ./a.out
>
> if you have got any questions regarding tail call stuff  i would be
> happy to help
>
> regards arnold
>
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