Assembly ".set" directive emitting symbol

Question

In some kernel-mode assembly source I have a line that looks like this:

; excerpt #1
.set __framesize, ROUND_TO_STACK(localvarsize)

(localvarsize is a parameter to a C-preprocessor macro, if you’re wondering.) I assume that __framesize is a compile-time variable that is usable in .if statements, and is then discarded. However, I find references to a symbol named __framesize in the symbol table and disassembly of my kernel. The symbol is defined (as output by nm -m) as such:

; excerpt #2
0000000000000000 (absolute) non-external __framesize

The usage of __framesize in compiler-generated assembly is as such:

; excerpt #3
movq %gs:__framesize, %rax
movq 0x140(%rax), %r15

Given what I understand of my compiler and my kernel, excerpt #3 should be emitted as movq %gs:0x140, %r15, and that code should work. (The code that is actually being emitted from the C as excerpt #3 is causing a triple fault on the second line.)

I have two questions:

1. Should this __framesize symbol be emitted into my binary by the assembler when used in this fashion? If possible, how can I suppress it?
2. Would this usage of __framesize cause a problem like what is discussed above?

I am using GAS assembler syntax and the Xcode 7.1.1 assembler, and a Mach-O output format, if it is useful.

Answer 1

The GNU as manual says that .set modifies the value(i.e. address) and/or type of an existing symbol. It's synonymous with .equ, so it can be used to set/modify assembler macro variable, or to mess around with symbols which are also labels.

If __framesize is showing up in the object file, then it's probably declared somewhere else.

Try looking at the disassembly output, to see what really happened.