How so? Your compiler produces a binary, binary gets loaded on a chip inside a weapon system. Is that not working on weapons? That wasn't so difficult.
I get the overall point, military spending as a huge funnel of free money to be thrown at research, of which not necessarily all would end up in actual combat ready systems. But you don't know which of your research would the uniforms throw away in government labs and which they would put on assembly lines and send to the field. Compilers are extraordinarily essential pieces of software that lie downstream of literally any source program, so even if your compiler was a research toy, it can be scavenged for that one algorithm or tricky data structure that can increase the efficiency of production compilers that produces the binaries that animate the killing machines on the field.
For example, here's [1] an article about how formal verification helps unmanned helicopters be more resistant to cyber attacks. I didn't even search for it specifically for this comment, I had it on open tabs for 3 days.
[1] https://m-cacm.acm.org/magazines/2018/10/231372-formally-ver...
My personal views on the matter tend towards what I would call a militaristic pacifism. I am pretty against actually going to war, but I am generally in favor of maintaining a large, powerful military, far more than most pacifists are. Having that military power gives you better leverage in international affairs, and more resistance against others' leverage. While you can theoretically get a similar position with strong moral high ground, the reality is that you need a truly impeccable history of sustaining the moral high ground for that position to not be eroded with whataboutism, a history that I think no political actor has ever attained.
Another perspective I will point out is that a decent fraction of "military expenditure" is not on actual military readiness. I've been on research grants that are coming out of the defense spending bucket, but I've never worked on anything remotely close to a weapon (e.g., one line item would have been building compilers, which is difficult to even maliciously twist into working on weapons).