Assembly comments affect generated code.

[jld]

Given this program:

fn main() {
        let x = @3;
        let mut a = 0;
        for uint::range(1, *x) |_i| {
                a += 1;
        }
        assert(a == 2);
}

compiling with -S -O yields an assembly file containing this, which is what's left of the uint::range loop (as output by grep -C2 inc):

        .align  16, 0x90
.LBB2_4:
        incq    %rcx
        cmpq    %rcx, %rax
        jne     .LBB2_4

But compiling with -c -O and disassembling shows a file where the loop has been entirely removed. However, compiling with -S -Zno-asm-comments -O is faithful to the -c output; and with --save-temps we can see that the bitcode is identical between -c and -S -Zno-asm-comments and nontrivially different (more than just the comments) for -S.

So there's something about the asm comments that makes LLVM think they have effects.

[nikomatsakis]

This is a known issue. However, the assembly comments are not generated unless you run with -S.

[nikomatsakis]

The problem is that the technique we use to inject comments is to insert an assembly block, which LLVM assumes could have arbitrary side-effects. There should be another way to do that...but I'm not sure what it is.

[jld]

The other problem is that a compiler is, more generally, not unreasonable in deleting operations which have no outputs that aren't unused and no effects on memory or other state. One is warned about this in the GCC asm statement documentation, for example (and in GCC's case, for extra confusion, a non-volatile asm with no outputs at all appears to be implicitly made volatile early in compilation, presumably under the assumption that unconditionally removing it was not the intended result).

Clearly there's some way of tracking which parts of the .text are which in order to have debug info, but that doesn't directly help with reading the assembly….

(Edited for what I hope is clarity.)

[nikomatsakis]

The defaults have been changed. We never generate assembly comments without -Z asm-comments. I am leaving the bug open because it'd be good to find a better way to inject debugging comments.

[thestinger]

This is still an issue.

[huonw]

Some searching yield two threads (1, 2) which suggest that one can attach a metadata string to the LLVM instruction and get the ASMPrinter to print it. This may mean some comments get lost entirely due to optimisations (e.g. DCE completely killing an instruction will presumably take its associated MDStrings with it).

[pnkfelix]

accepted for P-low.

[alexcrichton]

This option no longer exists, so closing.