LLVM 14 regression: std.leb128 miscompilation on riscv64

[andrewrk]

Zig Version: 0.10.0-dev.2896+fa40bddf2

Reproduce:

$ stage1/bin/zig test ../lib/std/std.zig -target riscv64-linux-none
Test [541/2199] leb128.test "std-riscv64-linux-none-Debug-bare-multi-default serialize ... FAIL (TestUnexpectedResult)
/home/andy/dev/zig/lib/std/leb128.zig:333:5: 0x57fb47 in leb128.test_write_leb128 (test)
    try testing.expect(w1_pos == bytes_needed);
    ^
Test [542/2199] leb128.test "std-riscv64-linux-none-Debug-bare-multi-default serialize ... FAIL (TestUnexpectedResult)
/home/andy/dev/zig/lib/std/leb128.zig:333:5: 0x57fb47 in leb128.test_write_leb128 (test)
    try testing.expect(w1_pos == bytes_needed);
    ^
2127 passed; 70 skipped; 2 failed.
error: the following test command failed with exit code 1:
qemu-riscv64 /home/andy/dev/zig/zig-cache/o/58c985004802103d620615fe2c55895a/test /home/andy/dev/zig/build-llvm14-debug/stage1/bin/zig
test...The following command exited with error code 1:
/home/andy/dev/zig/build-llvm14-debug/stage1/bin/zig test /home/andy/dev/zig/lib/std/std.zig --test-name-prefix std-riscv64-linux-none-Debug-bare-multi-default  --cache-dir /home/andy/dev/zig/zig-cache --global-cache-dir /home/andy/.cache/zig --name test -fno-single-threaded -target riscv64-linux-none -mcpu baseline_rv64 --test-cmd qemu-riscv64 --test-cmd-bin -I /home/andy/dev/zig/test -L /home/andy/local/llvm14-debug/lib -isystem /home/andy/local/llvm14-debug/include --zig-lib-dir /home/andy/dev/zig/lib --enable-cache 

Failing test cases:

zig/lib/std/leb128.zig

Lines 348 to 378 in aab1284

	test "serialize unsigned LEB128" {
	const max_bits = 18;
	comptime var t = 0;
	inline while (t <= max_bits) : (t += 1) {
	const T = std.meta.Int(.unsigned, t);
	const min = std.math.minInt(T);
	const max = std.math.maxInt(T);
	var i = @as(std.meta.Int(.unsigned, @typeInfo(T).Int.bits + 1), min);
	while (i <= max) : (i += 1) try test_write_leb128(@intCast(T, i));
	}
	}
	test "serialize signed LEB128" {
	// explicitly test i0 because starting `t` at 0
	// will break the while loop
	try test_write_leb128(@as(i0, 0));
	const max_bits = 18;
	comptime var t = 1;
	inline while (t <= max_bits) : (t += 1) {
	const T = std.meta.Int(.signed, t);
	const min = std.math.minInt(T);
	const max = std.math.maxInt(T);
	var i = @as(std.meta.Int(.signed, @typeInfo(T).Int.bits + 1), min);
	while (i <= max) : (i += 1) try test_write_leb128(@intCast(T, i));
	}
	}

Next steps:

1. Create LLVM IR test case
2. Use llvm-reduce to create reduction
3. Check if bug is present on trunk
4. File upstream bug report

[topolarity]

fn foo() void { }

test {
    var value = @as(u15, 0x4000); 
    _ = foo();
    if (@clz(u15, value) != 0) unreachable;
}

Does not reproduce on Zig 0.9.1 or current Zig master (with LLVM 13).

Reproduces with LLVM 14.0.5 and trunk. LLVM bug filed here: llvm/llvm-project#56457