On 24.09.2010 14:35, Jonas Maebe wrote: > > On 24 Sep 2010, at 11:48, Adrian Veith wrote: > >> Changing to pointers reduces the amount of multiplications for accessing >> the nth element in an array - if you compare the delphi code to th fpc >> code on assembler base, this is the main difference in both generated >> codes. > > Did you actually try replacing only the multiplications with lea's in > the assembler code generated by FPC (one lea to multiply by 5 and then > the times 4 during the load/store)? I did before posting my initial > reply because it also seemed to be the most logical explanation to me. > It turned out to be a red herring: > > With imull $20: > # iterations: 26662054 > no solution found > runtime: 10.75s > > With "lea (%reg,%reg,4),%reg" followed by "movl (%xxx,%reg,4),%yyy" > (not just for mov, but for every single memory expression that depends > on an "imull $20"): > # iterations: 26662054 > no solution found > runtime: 10.06s > > Kylix 3 (~ Delphi 6.5): > # iterations: 26662054 > no solution found > runtime: 6.65s
I must confess - I did not - but I will because that's interesting. Maybe this will behave different on different CPUs, because the picture is very different on my I7 compared to the older AMD. fpc: on i7: orig: 9s opt1: 7s = -22% opt2: 6s = -14% total -33% on AMD: orig: 17s opt1: 12s = -30% opt2: 7s = -41% total -58% so the older AMD suffers much more from the not optimized code than the newer i7. At the end they are almost on the same speed. same code with delphi (5): on i7: orig: 6s opt1: 5.5s = -8% opt2: 4.5s = -18% total -25% on AMD: orig: 9s opt1: 8s = -11% opt2: 6s = -25% total -33% Strange picture (I did all tests 3 times and took the medium). > >> Register allocation is on a comparable level for both versions. > > Delphi keeps the "Bar" pointer in a register, while FPC spills it to > the stack. Because Bar is used in most of the most-executed > statements, this has a huge impact. > you got me ;-) _______________________________________________ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
