Messages in this thread | | | Date | Fri, 25 Apr 2008 12:19:43 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 1/1] x86: fix text_poke |
| |
Mathieu Desnoyers wrote: > >>> b) there might be a jump into the middle of this instruction sequence? >>> >> If we change that, as discussed above, so the liveliness of ZF and of >> the %al register is still insured by leaving the mov and test >> instructions in place, we end up only modifying a single instruction and >> the problem fades away. We would end up changing a jne for a jmp. > > So, if we do is I propose here, we have to take into account this > question too. Any jump that jumps in the middle of this instruction > sequence would have to insure correct liveliness of %al and ZF. However, > since we just limited the scope of their liveliness, there are no other > code paths which can jump in the middle of our instruction sequence and > insure correct ZF and %al liveliness. >
I wanted to point out that this, in particular, is utter nonsense. Consider a sequence that looks something like this:
if (foo ? bar : imv_cond(var)) { blah(); }
An entirely sane transformation of this (as far as gcc is concerned), is something like:
cmpl $0,foo je 1f cmpl $0,bar jmp 2f 1: #APP movb var,%al /* This is your imv */ #NO_APP testb %al,%al 2: je 3f call blah 3:
Your code would take the movb-testb-je sequence and combine them, then we jump into the middle of the new instruction when jumping at 2!
There are only two ways to deal with this - extensive analysis of the entire flow of control, or telling the compiler exactly what is *actually* going on. The latter is the preferred way, obviously.
-hpa
| |