The only gain would be in terms of overhead, but it would depend. you would have to do it one of two ways: put the data you wish to check in a register, then jump, or just jump and write your code so that the value you wish to check will always be in the same register. I'm not really seeing any benafit to this, except for in embedded systems where the size of the code matters, and you would have to weigh that against the mov+jmp. I am interested to see Sina's take on this though. I'd love to start understanding more of how this code would be optomised by an assembler, how compilers do their optomization, etc.
Earlier this morning I finished up a bootloader, or part of one that boots to stage 2. I am finding that I am getting a lot better with the internals of things, and have a lot more understanding on how things work. I just have a lack of resources to kind of explain what I want to learn. So help/explainations/whatever would be really cool.
Thanks for taking a look at that, and for the info. On 1/16/2011 8:53 PM, Ken Perry wrote:
I don't think there is a really good way to speed this up. The fact is most assemblers will turn this into some really optimized machine code and with chips now days they can do multiple look ahead if statements so that this would almost happen in one pulse of a chips timing. I think Sina might be able to talk on the speed of this better than I but I can tell you the code is not bad. You have to understand Asm is different than most languages because its normal to have some repetitive code. You could make a block that tests for null and returns you to a passed in location so that way every time you wanted to test for null you jump to your test block and then if its null jump to end or jump back to the location in the code where you wanted to be but I don't see any gain in doing that. Ken -----Original Message----- From: programmingblind-bounce@xxxxxxxxxxxxx [mailto:programmingblind-bounce@xxxxxxxxxxxxx] On Behalf Of Littlefield, Tyler Sent: Sunday, January 16, 2011 9:56 PM To: programmingblind@xxxxxxxxxxxxx Subject: code optomization:any way to do this better? So I've been playing with assembly a lot lately, and was curious if there was a better way to do this. most importantly, the whole three branched if check (null, not null). section .text global _strcmp _strcmp: enter 0,0 ;we copy our arguments to EBX and ECX mov EBX, [EBP+8] mov ECX, [EBP+12] .loop: ;we need one value in a register mov EDX, [ECX] ;check for null termination cmp byte [EBX], 0 je .null jne .notnull ;we have a null termination. ;if the other string is null terminated, we jump to success. otherwise it fails because they obviously aren't equal. .null: cmp byte [ECX], 0 je .success jne .fail ;byte wasn't null, now we check for null on the other byte. ;if one is null, it's a fail because again they aren't equal. If it is not null, we do another check. .notnull: cmp byte [ECX], 0 ;not equal, we check for equalness between the two now. jne .check je .fail ;we check for equalness between the two bytes here. .check: cmp [EBX], EDX je .next jne .fail ;here we increase pointers and jump back up to the top of the loop. .next: inc EBX inc ECX jmp .loop ;strings compared fully .success: mov EAX,1 jmp .finish ;strings did not compare fully. .fail: mov EAX, 0 ;code cleanup. ;no need for a jmp, it just falls through. .finish: leave ret
-- Thanks, Ty __________View the list's information and change your settings at //www.freelists.org/list/programmingblind
