stage2: high memory use with trivial loops at comptime

[tauoverpi]

Zig Version

0.10.0-dev.3620+7d674d5fb

Steps to Reproduce

zig test bug.zig -fno-stage1

fn foo(comptime k: u32) u32 {
    var x: u32 = k;
    while (x +% k > x) : (x +%= 1) {}
    return x;
}

test {
    @setEvalBranchQuota(0xffff_ffff);
    comptime foo(5);
}

Expected Behavior

Constant memory use for the loop given that the loop is trivial.

Actual Behavior

Reached 20G in memory use (had to stop it before it reached OOM)

[nektro]

foo will infinite loop for any k other than zero. your high @setEvalBranchQuota tells the compiler to continue on

[tauoverpi]

@nektro the @setEvalBranchQuota is intentional to display the undesired behaviour and the loop condition doesn't actually matter here as the issue is the compiler filling up memory similar to #3461

[sagehane]

Since the reproducible code above has the same issue outlined in #12580, I propose a simpler version of foo to reproduce the issue:

fn foo() void {
    const u32_max = @import("std").math.maxInt(u32);

    @setEvalBranchQuota(u32_max);
    var x: u32 = 0;
    while (x != u32_max) : (x +%= 1) {}
}

[shwqf]

I think the potential reasons are:

a) Use .constant air to pass comptime value. e.g. calls addConstant in analyzeLoad, which leads to air_instructions become bigger and bigger.

b) infinite stacks.

b) is unavoidable, but a) can be improved...