x86_64 backend "emit MIR failed: CannotEncode" with comptime local in naked function

[190n]

Zig Version

0.15.0-dev.1271+bb2984673

Steps to Reproduce and Observed Behavior

repro.zig:

const saved_registers = [_][]const u8{ "rbx", "rbp", "r12", "r13", "r14", "r15" };

export fn entry() callconv(.naked) noreturn {
    inline for (saved_registers) |register| {
        asm volatile ("pushq %%" ++ register);
    }
}

$ zig build-obj -target x86_64-linux-gnu repro.zig
repro.zig:3:8: error: emit MIR failed: CannotEncode (Zig compiler bug)
export fn entry() callconv(.naked) noreturn {
~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The particular assembly doesn't seem to matter.

Expected Behavior

no error:

$ zig build-obj -target x86_64-linux-gnu -fllvm repro.zig
$ llvm-objdump -d repro.o

repro.o:        file format elf64-x86-64

Disassembly of section .text:

0000000000000000 <entry>:
       0: 53                            pushq   %rbx
       1: 55                            pushq   %rbp
       2: 41 54                         pushq   %r12
       4: 41 55                         pushq   %r13
       6: 41 56                         pushq   %r14
       8: 41 57                         pushq   %r15
       a: 0f 0b                         ud2

[mlugg]

The problem is actually just with having a named string local in a naked function:

export fn entry() callconv(.naked) noreturn {
    const r = "foo";
    _ = r;
}

$ ./stage4/bin/zig build-obj ~/repro.zig -target x86_64-linux --verbose-air -fstrip
# Begin Function AIR: repro.entry:
# Total AIR+Liveness bytes: 197B
# AIR Instructions:         5 (45B)
# AIR Extra Data:           8 (32B)
# Liveness tomb_bits:       8B
# Liveness Extra Data:      0 (0B)
# Liveness special table:   0 (0B)
  %0!= save_err_return_trace_index()
  %1!= dbg_stmt(2:5)
  %2!= dbg_var_val(<*const [3:0]u8, "foo">, "r")
  %3!= dbg_stmt(3:5)
  %4!= trap()
# End Function AIR: repro.entry

/home/mlugg/repro.zig:1:8: error: emit MIR failed: CannotEncode (Zig compiler bug)
export fn entry() callconv(.naked) noreturn {
~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

-fstrip avoids the bug. The bug appears to reproduce if r is a non-comptime-only pointer; so strings do it, as does const r: *const u32 = &0;.