Author: Ed Schröder
Date: 01:16:54 01/14/02
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On January 13, 2002 at 23:36:19, Eugene Nalimov wrote: >Can you please send me the function that was so badly compiled (probably via >e-mail)? I'd like to find where VC screwed up. It's too late to fix it for VC7, >but probably we can do it for VC7.x. > >Eugene Screwed up is a big word, ASM being being just 30% faster than C is a very good performance I would say. By head I remember the following cases: #1. a=b; c=d; The compiler will output something like: mov EAX,b mov a,EAX mov EAX,d mov c,EAX Wheras it should generate: mov EAX,b mov EBX,d mov a,EAX mov c,EBX #2. Always these unavoidable MOVSX and MOVZX instructions. No compiler can optimize this because it is impossible, only the ASM programmer knows what it is allowed under the circumstances. #3. Register use, same story as (2). I for instance use EBP and even ESP when I am short on registers. #4. "char" use in MSVC, for instance: char a1,a2,a3,a4,a5,a6,a7,a8; Will NOT produce the 8 characters as a sequential memory block. So in case I want to zero the 8 bytes I will be forced to write 8 instructions. Some other compilers do generate a sequential memory block so you can redefine a1 and a5 as 32-bit and with 2 instructions zero them. This is pretty crucial in a chess program, at least in mine, also because I have to "stack" many stuff when going one ply deeper in the tree or when climbing back. #5. Special stuff, no compiler is able to recognize as only the ASM programmer knows. I recently posted an example how to use the "indirect jump" the processor is offering you when for instance generating moves. So it is not about bugs, it is more why no compiler will be ever able to beat an experienced ASM programmer. However I do think that there is space for improvement in the (1) and (4) case, maybe even on (3). Ed >On January 13, 2002 at 18:51:02, Ed Schröder wrote: > >>On January 13, 2002 at 16:29:21, Tom Kerrigan wrote: >> >>>On January 13, 2002 at 07:05:02, Ed Schröder wrote: >>> >>>>I have to disagree, I have a MSVC6 version of Rebel and it runs 30% slower than >>>>the ASM version. >>> >>>What do you attribute this difference to? Is it simply not possible to write C >>>that produces the same assembly as your hand-written code? Or do you take >>>certain liberties in the C code (perhaps in the same of readability?) that's >>>slowing things down? >>> >>>-Tom >> >>Just have a look at the ASM code MSVC6 produces, it often is bad stuff. By >>re-writing (optimizing) this "bad ASM stuff" I got my +30%. >> >>One ambiguous remark, don't believe everthing commercials are telling you :) >> >>Ed
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