3 ms·

In this particular example, why not just PXOR XMM7 XMM7 before using XMM7 to zero anything? That way the compiler doesn't have to assume that XMM7 is zero,

In this particular example, why not just PXOR XMM7 XMM7 before using XMM7 to zero anything? That way the compiler doesn't have to assume that XMM7 is zero, it can know. Yes it's an extra instruction but XORing a register with itself is such a common metaphor for zeroing that register that CPU designers try to make it fast. Edit: Just noticed that Veliladon essentially made the same comment herein and explained the reason why it's not done this way.

puffoflogic 4y ago
It's not just that. If you can't trust the ABI then everything else is wrong too. Everything. Not just zeroing registers. It only just so happens that in this case XMM7 is used for zeroing, but it could be used to save a variable across a function call. Then there is no trick to get it set back to the right value. The ABI is not optional, or best effort, or best practice, or any other BS that passes in the ordinary world. It is just as required as the correct operation of instructions (e.g., add should actually add things, mul should actually multiply them, and so on).
mrkeen 4y ago
My reading of it was that the bug was caused by XMM7 not being restored to its previous value. E.g. on Linux, functions should restore the values of ebx, esi, edi, ... once they're done with them. The article says (on Windows) that XMM7 needs to be restored too. If you can't trust one register being preserved (per the ABI), then you really can't trust the values of any registers.
- dreamcompiler 4y ago
  Fair point, which is why I used the phrase "this particular example." In fact the compiler's lazy assumption about XMM7 was the key to determining that the ABI was being violated. It would have taken longer to figure this out if the compiler had been doing things my way, because then a failure of the callee to preserve XMM7 wouldn't have mattered.
  - mjg59 4y ago
    You could avoid the crash in this specific case. But this crash was triggered by an assertion failure. The same code could be corrupting XMM7 in other situations, which could result in much more subtle bugs.
  - toast0 4y ago
    What's the point of having an ABI if you can't assume it's being used? If you don't want to trust any callees, you could use an ABI with all registers caller saves, but I don't think any mainstream ABIs are like that. There's a balance where having some registers be caller saved and others callee saved means a lot less saving required.

puffoflogic 4y ago

It's not just that. If you can't trust the ABI then everything else is wrong too. Everything. Not just zeroing registers. It only just so happens that in this case XMM7 is used for zeroing, but it could be used to save a variable across a function call. Then there is no trick to get it set back to the right value. The ABI is not optional, or best effort, or best practice, or any other BS that passes in the ordinary world. It is just as required as the correct operation of instructions (e.g., add should actually add things, mul should actually multiply them, and so on).

mrkeen 4y ago

My reading of it was that the bug was caused by XMM7 not being restored to its previous value. E.g. on Linux, functions should restore the values of ebx, esi, edi, ... once they're done with them. The article says (on Windows) that XMM7 needs to be restored too. If you can't trust one register being preserved (per the ABI), then you really can't trust the values of any registers.

dreamcompiler 4y ago
Fair point, which is why I used the phrase "this particular example." In fact the compiler's lazy assumption about XMM7 was the key to determining that the ABI was being violated. It would have taken longer to figure this out if the compiler had been doing things my way, because then a failure of the callee to preserve XMM7 wouldn't have mattered.
- mjg59 4y ago
  You could avoid the crash in this specific case. But this crash was triggered by an assertion failure. The same code could be corrupting XMM7 in other situations, which could result in much more subtle bugs.
- toast0 4y ago
  What's the point of having an ABI if you can't assume it's being used? If you don't want to trust any callees, you could use an ABI with all registers caller saves, but I don't think any mainstream ABIs are like that. There's a balance where having some registers be caller saved and others callee saved means a lot less saving required.