[LLVMdev] Intel asm syntax and variable names

[Reid Kleckner]

Suppose I have a global variable named 'EAX'. How do Intel assemblers
normally escape register names to access such a global variable?

On Thu, Jul 23, 2015 at 1:42 AM, Yatsina, Marina <marina.yatsina at intel.com>
wrote:

>  Hi all,
>
>
>
> I’ve encountered an issue with x86 Intel asm syntax when using certain
> variable names.
>
>
>
> If you look at the following example, where I try to do a mov to a memory
> location named “flags2”, llvm- mc works fine:
>
>
>
> >cat test_good.s
>
> mov eax, flags2
>
> >llvm-mc.exe -x86-asm-syntax=intel test_good.s -o -
>
>         .text
>
>         movl    flags2, %eax
>
>
>
> But if the memory location is named “flags”, llvm-mc fails:
>
>
>
> >cat test_bad.s
>
> mov eax, flags
>
> >llvm-mc.exe -x86-asm-syntax=intel test_bad.s -o -
>
> test_bad.s:1:1: error: invalid operand for instruction
>
> mov eax, flags
>
> ^
>
>         .text
>
>
>
> After investigation, I saw that the memory location named “flags” was
> matched to the EFLAGS register in the MatchRegisterName() function in the
> generated X86GenAsmMatcher.inc.
>
>
>
> case 'f': // 1 string to match.
>
>       if (memcmp(Name.data()+1, "lags", 4))
>
>         break;
>
>       return 25;      // "flags"
>
>
>
> So basically, what I’m seeing with “flags” (which should be a legit
> variable name) is that the X86AsmParser creates a reference to an implicit
> register instead of a reference to memory.
>
> There are additional issues here as well - what if we compile to SSE, but
> use a variable named “ZMM0” which is a register in AVX-512? Should this be
> allowed?
>
>
>
> We probably need some way to mark the registers (using attributes or
> predicates?) so that we’d know which ones are part of the legal set of
> registers that can be referenced in the architecture we’re compiling too.
>
> Do you think this is a good approach?
>
>
>
> Thanks,
>
> Marina
>
>
>
>
>
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