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Please restore our registers when you’re done with them

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Re: Please restore our registers when you’re done with them

#21

Post by pjc50 »

323 wrote: post #15

Sounds like there should be an option when you write assembly code to tell the compiler "please save/restore any register that I'm modifying in this asm code according to the target you are compiling for"

That kind of feature is very .. un-assembler. You certainly could, but the assembler doesn't keep track of any of the relevant information, so it would be a larger feature than you expect. It doesn't know the calling convention. It doesn't have a map of registers dirtied by which instructions. It doesn't do reachability analysis, so it doesn't even necessarily know what's "in the function".

Re: Please restore our registers when you’re done with them

#22

Post by rocqua »

To what degree is this possible to check statically?

It feels like at least simple breaks of the ABI rules like this can be detected somewhat statically. The author already started with a very simple and incomplete version.

In general, I wonder, are there any (many?) static analyzers for assembled binaries.

Re: Please restore our registers when you’re done with them

#23

Post by pjc50 »

brucedawson wrote: post #20
electroly wrote: post #10

Earlier quoted context omitted.

Those PUSH_XMM/POP_XMM macros appear to be Windows-only; I think they expand to nothing on other platforms because they contain their own guard for Windows internally. If that's the case, the call sites don't need to guard for it. I'm guessing that obeying this calling convention is the purpose of those macros. https://github.com/cisco/openh264/blob/db956674bbdfbaab5acdd... https://github.com/cisco/openh264/blob/db95…

The webrtc fix was thematically similar in that the programmer declared what registers were trashed and then the compiler knows which registers need to be saved. I'm not sure why the compiler doesn't notice when registers are used without being declared as being trashed - I'm really not an expert at _writing_ assembly language.

Traditionally, an assembler neither knows nor cares about such information, it just turns lines of assembly into bytes and does some address fixup for you.

Re: Please restore our registers when you’re done with them

#24

Post by dfox »

323 wrote: post #15

Sounds like there should be an option when you write assembly code to tell the compiler "please save/restore any register that I'm modifying in this asm code according to the target you are compiling for"

THat is essentially how inline assembly in GCC works. You have to declare which registers you are changing and in which registers you expect input and which contain results from your inline assembly block.

This for obvious reasons does not work if you have separate compilation unit written in assembly, then you have to follow the ABI.

Re: Please restore our registers when you’re done with them

#25

Post by Joker_vD »

pjc50 wrote: post #21
323 wrote: post #15

Sounds like there should be an option when you write assembly code to tell the compiler "please save/restore any register that I'm modifying in this asm code according to the target you are compiling for"

That kind of feature is very .. un-assembler. You certainly could, but the assembler doesn't keep track of any of the relevant information, so it would be a larger feature than you expect. It doesn't know the calling convention. It doesn't have a map of registers dirtied by which instructions. It doesn't do reachability analysis, so it doesn't even necessarily know what's "in the function".

Doesn't GCC inline assembly actually track that info, with the "clobber" section?

Re: Please restore our registers when you’re done with them

#26

Post by flanfly »

323 wrote: post #15

Sounds like there should be an option when you write assembly code to tell the compiler "please save/restore any register that I'm modifying in this asm code according to the target you are compiling for"

I not sure whether that satire. In case its not, most languages that allow inline assembly (like C) have an optional "clobber list" argument that tells the dataflow analysis of the compiler that your assembly snippet overwrites certain registers [1]. Inline assembly doesn't have target specific clobber lists because it's assumed that the code only works on one target and the programmer has to take care of making it work.

1: https://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO....

Re: Please restore our registers when you’re done with them

#27

Post by masklinn »

Joker_vD wrote: post #25
pjc50 wrote: post #21

Earlier quoted context omitted.

That kind of feature is very .. un-assembler. You certainly could, but the assembler doesn't keep track of any of the relevant information, so it would be a larger feature than you expect. It doesn't know the calling convention. It doesn't have a map of registers dirtied by which instructions. It doesn't do reachability analysis, so it doesn't even necessarily know what's "in the function".

Doesn't GCC inline assembly actually track that info, with the "clobber" section?

Yes, but it's not a property of the assembly (or assembler), it's a necessity for the compiler to correctly codegen around the inline assembly.

Historically, assemblers have been really dumb, so ABI is not a thing they'd track, especially as... I don't think they know what functions are? So while they can notice call/ret, they have no knowledge of a label being a jump or call target per-se, do they?

So you'd need an assembly-like language to encode this sort of information.

Re: Please restore our registers when you’re done with them

#29

Post by matthews2 »

323 wrote: post #15

Sounds like there should be an option when you write assembly code to tell the compiler "please save/restore any register that I'm modifying in this asm code according to the target you are compiling for"

Maybe we need some kind of higher level language that gets boiled down into assembly.

Re: Please restore our registers when you’re done with them

#30

Post by Veliladon »

bombcar wrote: post #3

It’s interesting that there is a zero stored in a register and used for hours - is that significantly faster than just using some actual zero each time? Perhaps CPUs need a “always zero” register or some similar menomic to help harden.

Intel has never really needed to have a zero register because xor register, register as a zeroing idiom is so fast and so recognized that Intel have optimized the hell out of it. In Sandy Bridge and onward it doesn't even go through an execution port, even for the vector registers.

The problem is really whether to indulge bad programmers who don't respect the ABI at the cost of a minimal sliver of performance (even though it's not taking up an execution port the extra instruction still takes up cache space, bandwidth, and decode). Yeah they should probably zero the register before they zero the pointer but they shouldn't have to if other people respected the ABI.

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