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Re: PATCH: Add --alt-nops=short|long to x86/x86-64 assemblers

Hi H. J.

I am not familiar with the x86 instruction set, so please can you explain why it is necessary to have these two different versions of the nop and why the user has to select one ? ie if the space to be padded is up to 10 bytes why can't the assembler just use the "short" version automatically and if it is longer than 10 bytes use the "long" version ?

x86/x86-64 assemblers need to fill the text section from 1 to 15 bytes for alignment. We have simple nop instructions for 1 to 10 bytes. For 11 to 15 bytes, we can add 0x66 prefix repeatedly to 10 byte nop to get 11-15byte nops. However some processors prefer simple nops. That is, 3 0x66 prefixes on the 10 byte nop are slower on those processors than a 6 byte nop + a 7 byte nop.

Does the assembler not know the target processor variant ? ie is the assembler unable to make an informed choice of which type of nop to use ?

User can use --alt-nops=short to avoid repeated 0x66 prefixes.
Maybe we can use something other than short|long. But I don't
have a better name.

How about "--allow-long-nop-sequences" and its inverse "--no-allow-long-nop-sequences" ? The default would presumably be to allow the long sequences since this is faster on more modern processors, yes ?

I would also suggest that you extend the description in your patch to c-i386.texi to cover the explanation you just gave me, unless you are sure that all x86 assembler programmers would know about these two types of nop.


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