On Tue, 2005-11-01 at 17:30 +0100, Florian Weimer wrote:
use C control constructs rather than gotos.
With GCC version 4, this will have no effect because the gimplifier converts everything to goto-style anyway.
Felix generates C with gotos. The result is FASTER than native C using gcc 4.0 on x86_64. http://felix.sourceforge.net/current/speed/en_flx_perf_0005.html C code: 4: int Ack(int M, int N) { 5: if (M==0) return N +1; 6: else if(N==0) return Ack(M-1,1); 7: else return Ack(M-1, Ack(M,N-1)); 8: } 9: It is known gcc is correctly optimising tail calls here. Felix: 5: fun ack(x:int,y:int):int => 6: if x == 0 then y + 1 7: elif y == 0 then ack(x-1, 1) 8: else ack(x-1, ack(x, y-1)) 9: endif 10: ; Felix generated C(++) code -- compiled with same options: int FLX_REGPARM _i1860_f1301_ack( int _i1864_v1303_x, int _i1865_v1304_y) { _us2 _i1867_v1799_ack_mv_74; _us2 _i1868_v1821_ack_mv_84; start_1828:; _i1867_v1799_ack_mv_74 = _i1864_v1303_x==0 ; if(!(_i1867_v1799_ack_mv_74==1))goto _1797; return _i1865_v1304_y+1 ; _1797:; _i1868_v1821_ack_mv_84 = _i1865_v1304_y==0 ; if(!(_i1868_v1821_ack_mv_84==1))goto _1798; _i1865_v1304_y = 1; _i1864_v1303_x = _i1864_v1303_x-1 ; goto start_1828; _1798:; _i1865_v1304_y = _i1860_f1301_ack(_i1864_v1303_x, _i1865_v1304_y-1 ); _i1864_v1303_x = _i1864_v1303_x-1 ; goto start_1828; } [The FLX_REGPARM says __attribute__((regparm(3)) when gcc is the compiler for i386 .. but it has no effect on x86_64] AFAICS gcc 4.x generates much better code if you just use gotos everywhere instead of C control structures. I have no real idea why the Felix generated C is faster. Two guesses: (a) the two 'mv' variables declared at the top are optimised away, so the Felix version is only using 3 words of stack. (b) the "parallel assigment in tail calls optimisation" is saving one word on the stack (evaluating y before x saves a temporary across the non-tail recursion). but I don't really know. -- John Skaller <skaller at users dot sf dot net> Felix, successor to C++: http://felix.sf.net