On 27 April 2011 20:01, Denys Rtveliashvili
The lack of expected magic is in the assembler code: -------------------
addq $16,%r12 cmpq 144(%r13),%r12 ja .Lcz1 movl $1117,%ecx movl $1113,%r10d movl $1111,%r11d movq 7(%rbx),%rax cqto idivq %r11 cqto idivq %r10 cqto idivq %rcx movq $ghczmprim_GHCziTypes_Izh_con_info,-8(%r12) movq %rax,0(%r12) leaq -7(%r12),%rbx addq $8,%rbp jmp *0(%rbp)
------------------- Question: can't it use cheap multiplication and shift instead of expensive division here? I know that such optimisation is implemented at least to some extent for C--. I suppose it also won't do anything smart for expressions like a*4 or a/4 for the same reason.
There isn't really any optimisation done on Cmm and the native code generator doesn't do much optimisation itself, hence you get the more direct forward translation. This kind of code is where the LLVM backend does well in comparison. I haven't tried benchmarking the performance of -fasm vs -fllvm for this code but if you eyeball the assembly code produced by -fllvm then you'll see it uses shifts and other magic. Cheers, David