[BUG] While loop logic causes seg fault.

johnsoez4 commented 2 weeks ago

Bug description

This issue started a few days ago with Mojo nightly version 2024.5.1002. Edit 2024-05-28:

mojo 2024.5.822 (2cfed0ad) = OK
mojo 2024.5.1002 (83c7e1be) = KO (issues started)

In the code shown below, function app_run causes a seg fault, but app_close alone compiles and works properly. The logic to exit each while loop is essentially identical but inverted. I presume that both functions should compile and work properly. Important: I just confirmed that this issue does NOT exist with Mojo stable version 24.3.0 (9882e19d). stack_dump.txt

Steps to reproduce

fn main() raises:
    app_close()
    app_run()

fn app_close():
    # This compiles and works.
    var app_close = False
    var loops = 10
    while not app_close:
        loops -= 1
        if loops == 0:
            app_close = True
    print("app_close: Success!")

fn app_run():
    # This does not compile (seg fault).
    var app_run = True
    var loops = 10
    while app_run:
        loops -= 1
        if loops == 0:
            app_run = False
    print("app_run: Success!")

System information

Ubuntu: 24.04 LTS
Python: 3.12.3
Mojo Nightly: 2024.5.1619 (bd52708e)
Modular: 0.7.4 (df7a9e8b)

johnsoez4 commented 1 week ago

No problem (confirmed OK on 2024-05-28)

mojo 2024.5.822 (2cfed0ad) OK

Problem confirmed:

mojo 2024.5.1002 (83c7e1be)
mojo 2024.5.1805 (341542be)
mojo 2024.5.1905 (46b7e7ee)
mojo 2024.5.2012 (fb60fe4f)
mojo 2024.5.2119 (79ef45f6)
mojo 2024.5.2205 (d3cacc7e)
mojo 2024.5.2305 (1e5d508e)
mojo 2024.5.2305 (1e5d508e)
mojo 2024.5.2414 (39359abc)
mojo 2024.5.2605 (9c328b12)
mojo 2024.5.2805 (87564503)
mojo 2024.5.2912 (6861a424)
mojo 2024.5.3005 (0c465b6a)
mojo 2024.5.3112 (e60dc71f)

JoeLoser commented 1 week ago

@ematejska @jopamer also asked on discord at https://discord.com/channels/1087530497313357884/1224434323193594059/1242260289060077589

johnsoez4 commented 3 days ago

@ematejska, @JoeLoser , @mzaks, @jopamer, @rd4com, @jdiggins

Here's an interesting clue to the mystery. It works when the line while app_run: is changed to while not not app_run:.

fn app_run():
    # This does not compile (seg fault).
    var app_run = True
    var loops = 10

    while not not app_run:  # This works.
    # while app_run:  # This does not work.
        loops -= 1
        if loops == 0:
            app_run = False
    print("app_run: Success!")

JoeLoser commented 3 days ago

This is a compiler bug from one of our passes. It's hitting an assertion in our compiler stack inside LLVM:

Assertion failed: (detail::isPresent(Val) && "dyn_cast on a non-existent value"), function dyn_cast, file Casting.h, line 662.FYI

FYI @jeff

ematejska commented 3 days ago

Thanks for filing! We'll take a look.

ematejska commented 3 days ago

Duplicate filed here: https://github.com/modularml/mojo/issues/2821

Reduced use-case from there:

fn main():
    var loop = True
    while loop:
        loop=False

modularml / mojo