Bit operations causes compiler to hang

[schmee]

The following example hangs the compiler when compiling with zig build-exe test.zig:

fn foo(x: u32) void {
    const p: u48 = (x >> 1) & 1;
}

pub fn main() void {
    foo(1);
}

If you change p to const p = (x >> 1) & 1 (i.e remove the explicit cast to u48) it compiles immediately. If you change the cast to u32 or u64 it compiles. All numbers I tried between 32 and 64 also caused the hang. It seems very specific to these operators: I've tried replacing << and & with various combinations but only this one causes the error. If you inline the function call, it also compiles.

Versions:

➜  zig version
0.3.0+7fc99c33

macOS Mojave 10.14.3

[andrewrk]

Nice find. I'm actually able to reproduce this using clang-7 and clang-8 and the LLVM IR file that Zig produces from this source. So it's pretty clearly an LLVM bug. I'm making a bug report now.

[andrewrk]

Upstream bug report: https://bugs.llvm.org/show_bug.cgi?id=40676

[andrewrk]

It's unlikely this will be fixed in the upcoming llvm 8.0.0 release, since it's already at Release Candidate 2. However I will make it a goal to submit an upstream bug fix before they make the llvm 9 branch.

[schmee]

I don't know if this is useful or not, but the example file you posted doesn't hang for me on macOS:

➜  clang -c test.ll
warning: overriding the module target triple with x86_64-apple-macosx10.14.0 [-Woverride-module]
1 warning generated.
➜  clang --version
Apple LLVM version 10.0.0 (clang-1000.11.45.5)
Target: x86_64-apple-darwin18.2.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

[andrewrk]

This is fixed in llvm trunk but not llvm 8.0.rc2. I don't plan to do the bisect necessary to find which commit fixed it, so unless someone else wants to do that before 8 is released, this will remain an open issue until LLVM 9, which corresponds to Zig 0.5.0.