Distinguishing memory from constant in GNU as .intel_syntax

问题

I have an instruction written in Intel syntax (using gas as my assembler) that looks like this:

mov rdx, msg_size
...
msg: .ascii "Hello, world!\n"
     .set msg_size, . - msg

but that mov instruction is being assembled to mov 0xe,%rdx, rather than mov $0xe,%rdx, as I would expect. How should I write the first instruction (or the definition of msg_size) to get the expected behavior?

回答1:

In GAS .intel_syntax noprefix mode:

• OFFSET symbol works like AT&T $symbol. This is somewhat like MASM.
• symbol works like AT&T symbol (i.e. a dereference).
• [symbol] is always an effective-address, never an immediate, in GAS and NASM/YASM. LEA doesn't load from the address, but it still uses the memory-operand machine encoding. (That's why it uses the same syntax).

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Tip: if you know the AT&T syntax, or the NASM syntax, for something, use that to produce the encoding you want and then disassemble with objdump -Mintel to find out the right syntax for .intel_syntax noprefx.

BTW, no open-source projects that I'm aware of contain GAS intel_syntax source code. If they use gas, they use AT&T syntax. Otherwise they use NASM/YASM. (You sometimes also see MSVC inline asm in open source projects).

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GNU as .intel_syntax noprefix mode is somewhat like MASM, except there's no magic that depends on how symbols are defined.

mov rdx, symbol is always a load. A symbol's value is its address. i.e. using .set creates (or modifies) a symbol that behaves exactly the same as putting a label on something.

(unless symbol was defined as an assemble-time constant with symbol=123, or .equ symbol, 123, then it's a mov-immediate.)

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mov rdx, OFFSET symbol will assemble to mov r/m64, imm32. I forget if intel-syntax mode lets mov assemble to movabs r64, imm64 with large operands, but that's irrelevant because static data/code addresses are always in the low 2GiB of virtual address space, so they fit in 32-bit sign-extended immediates.

(This makes it safe to write mov edx, OFFSET symbol, and in fact you should always do that or use lea rdx, [rip + symbol], never sign-extended 32-bit immediate unless you're writing code that will be loaded into the high 2GB of virtual address space.)

来源：https://stackoverflow.com/questions/39355188/distinguishing-memory-from-constant-in-gnu-as-intel-syntax