Boolean optimization no longer works

[pedantic79]

I'm unsure if this is a bug or not. The following code used to perform just as well as if there were no let statements. However, sometime since last year, that has changed.

pub fn is_leap_year1(year: i32) -> bool {
    let div_4 = year % 4 == 0;
    let div_100 = year % 100 == 0;
    let div_400 = year % 400 == 0;
    div_4 && !(div_100 && !div_400)
}

Starting with rust 1.33, all the modulos are always computed. While in 1.32, it would stop processing if the year was not divisible by 4. Using compiler explorer, I was able to find when is_leap_year assembly output changed. (link to godbolt with assembly output).

I wanted to make sure it wasn't a change in LLVM, so I looked at the llvm-ir. The same logic between the llvm-ir and assembly existed. (link to godbolt with llvm-ir output)

Next was to determine exactly when this change occurred, nightly-2018-12-24 had the optimization and nightly-2018-12-26 did not. Unfortunately, nightly-2018-12-25 won't install via rustup. I was however able to manually download rustc and std, and was able to verify that it is broken then.

root@2c92852f7036:~# rustc --version
rustc 1.33.0-nightly (f960f377f 2018-12-24)
root@2c92852f7036:~# rustc --crate-type=lib -C opt-level=3 --emit=llvm-ir leap.rs
root@2c92852f7036:~# sed -n -e '/is_leap_year1/,/}/ p' leap.ll
; leap::is_leap_year1
; Function Attrs: norecurse nounwind nonlazybind readnone uwtable
define zeroext i1 @_ZN4leap13is_leap_year117ha0a43b53679ed341E(i32 %year) unnamed_addr #0 {
start:
  %0 = and i32 %year, 3
  %1 = icmp eq i32 %0, 0
  %2 = srem i32 %year, 100
  %3 = icmp ne i32 %2, 0
  %4 = srem i32 %year, 400
  %5 = icmp eq i32 %4, 0
  %. = or i1 %5, %3
  %_0.0 = and i1 %1, %.
  ret i1 %_0.0
}

So some commit between these two nightly removed this optimization:

• rustc 1.33.0-nightly (ddab10a69 2018-12-23)
• rustc 1.33.0-nightly (f960f377f 2018-12-24)