Messages in this thread | | | Subject | Re: [PATCH] x86: pad assembly functions with INT3 | From | "H. Peter Anvin" <> | Date | Fri, 11 May 2018 11:53:40 -0700 |
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On 05/10/18 09:39, David Laight wrote: > From: Alexey Dobriyan >> Sent: 07 May 2018 22:38 >> >> Use INT3 instead of NOP. All that padding between functions is >> an illegal area, no legitimate code should jump into it. >> >> I've checked x86_64 allyesconfig disassembly, all changes looks sane: >> INT3 is only used after RET or unconditional JMP. > > I thought there was a performance penalty (on at least some cpu) > depending on the number of and the actual instructions used for padding. > > I believe that is why gcc generates a small number of very long 'nop' > instructions when padding code. >
There is a performance penalty for using NOP instructions *in the fallthrough case.* In the case where the padding is never supposed to be executed, which is what we're talking about here, it is irrelevant.
I thought I had filed a gcc enhancement request, but I can't find it now, so I just filed this:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85751
-hpa
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