[PATCH] D53540: [COFF, ARM64] Implement support for SEH extensions __try/__except/__finally

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.


================
Comment at: lib/Target/AArch64/AArch64AsmPrinter.cpp:697
     break;
+    case AArch64::MOVMCSym: {
+    MCSymbol *Sym = MI->getOperand(1).getMCSymbol();
----------------
rnk wrote:
> mgrang wrote:
> > @rnk Yes, you are correct that the assert here will fail is the localrecover comes before localescape.
> > I have tried to model this similar to X86 but the problem is AArch64 cannot do an ADRP on an MCSymbol. So I wasn't sure how to materialize this in the assembler.
> > Could you give me some idea on how this can be done?
> I guess I've been assuming that you could assemble something like this in a .s file for aarch64, like you can for x86:
> ```
> main:
>   movl $.Lasdf, %eax
>   ret
> .Lasdf = 1234
> ```
> 
> I would assume that the aarch64 equivalent would be:
> ```
> main:
>   mov d0, #.Lasdf
>   ret
> .Lasdf = 1234
> ```
> 
> But that doesn't assemble. Does gnu as support that? I don't have an aarch64 binutils build. Maybe llvm-mc should allow mov/movk to accept a symbolic label operand.
> 
> I don't think we would want to use ADRP, IIRC that's for materializing some PC-relative address, but obviously this isn't that kind of symbol.
> Does gnu as support that?

"mov" is pseudo-instruction, but it will expand to at most one instruction (a movz, a movn, or an orr); this is defined in the AArch64 architecture reference.  There is no variable-length pseudo-instruction; if you need a larger value, you have to write out a movz/movk sequence explicitly.

I guess the stack size is currently limited to 256MB on Windows because of the way we emit the prologue on Windows (that's the limit for the alloc_l unwind opcode).  We could maybe teach LLVM to use multiple alloc_l operations, but it's probably reasonable to assume the stack for a function is less than 4GB, so we could emit just two instructions (movz+movk).

For value less than 2^32, gas supports something like the following:

```
movz x0, #:abs_g1:.Lasdf
movk x0, #:abs_g0_nc:.Lasdf
.Lasdf = 0x12345678
```

If you try this with LLVM, you currently get an error "no resolvable MOVZ/MOVK fixups supported yet", but I think that's straightforward to fix.  (LLVM supports the syntax in general, just not the case where the value is defined in the same object file.)


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  https://reviews.llvm.org/D53540/new/

https://reviews.llvm.org/D53540