[LLVMdev] Intel asm syntax and variable names

[Yatsina, Marina]

Microsoft assembler treats mov to EAX as a register, even if there is a global memory also named EAX – meaning the register takes precedence.
But here I have a bit of a different situation – I have a global variable, which name happens to match an implicit register or a register that does not exist in the current arch, just in future ones. Microsoft assembler treats these cases as memory locations, llvm treats them as registers, causing compilation errors.


From: Reid Kleckner [mailto:rnk at google.com]
Sent: Thursday, July 23, 2015 18:54
To: Yatsina, Marina
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Intel asm syntax and variable names

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<mailto: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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