Auto merge of #87794 - bonega:enum_niche_prefer_zero, r=nagisa

Enum should prefer discriminant zero for niche

Given an enum with unassigned zero-discriminant, rust should prefer it for niche selection.
Zero as discriminant for `Option<Enum>` makes it possible for LLVM to optimize resulting asm.

- Eliminate branch when expected value coincides.
- Use smaller instruction `test eax, eax` instead of `cmp eax, ?`
- Possible interaction with zeroed memory?

Example:
```rust

pub enum Size {
    One = 1,
    Two = 2,
    Three = 3,
}

pub fn handle(x: Option<Size>) -> u8 {
    match x {
        None => {0}
        Some(size) => {size as u8}
    }
}
```
In this case discriminant zero is available as a niche.

Above example on nightly:
```asm
 mov     eax, edi
 cmp     al, 4
 jne     .LBB0_2
 xor     eax, eax
.LBB0_2:
 ret
```

PR:
```asm
 mov     eax, edi
 ret
```

I created this PR because I had a performance regression when I tried to use an enum to represent legal grapheme byte-length for utf8.

Using an enum instead of `NonZeroU8` [here](https://github.com/bonega/yore/blob/d683304f5dfe2e99f769e6ab8adf8d60a0d1d9b3/src/internal/decoder_incomplete.rs#L90)
resulted in a performance regression of about 5%.
I consider this to be a somewhat realistic benchmark.

Thanks to `@ogoffart` for pointing me in the right direction!

Edit: Updated description

2 files changed, 39 insertions, 0 deletions

diff --git a/src/test/assembly/niche-prefer-zero.rs b/src/test/assembly/niche-prefer-zero.rs
new file mode 100644
index 00000000000..0ab37a618da
--- /dev/null
+++ b/src/test/assembly/niche-prefer-zero.rs
@@ -0,0 +1,25 @@
+// Check that niche selection prefers zero and that jumps are optimized away.
+// See https://github.com/rust-lang/rust/pull/87794
+// assembly-output: emit-asm
+// only-x86
+// compile-flags: -Copt-level=3
+
+#![crate_type = "lib"]
+
+#[repr(u8)]
+pub enum Size {
+    One = 1,
+    Two = 2,
+    Three = 3,
+}
+
+#[no_mangle]
+pub fn handle(x: Option<Size>) -> u8 {
+    match x {
+        None => 0,
+        Some(size) => size as u8,
+    }
+}
+
+// There should be no jumps in output
+// CHECK-NOT: j
diff --git a/src/test/ui/enum-discriminant/niche-prefer-zero.rs b/src/test/ui/enum-discriminant/niche-prefer-zero.rs
new file mode 100644
index 00000000000..f20607a8903
--- /dev/null
+++ b/src/test/ui/enum-discriminant/niche-prefer-zero.rs
@@ -0,0 +1,14 @@
+// Check that niche selection prefers zero.
+// See https://github.com/rust-lang/rust/pull/87794
+// run-pass
+#[repr(u8)]
+pub enum Size {
+    One = 1,
+    Two = 2,
+    Three = 3,
+}
+
+fn main() {
+    // check that `None` is zero
+    assert_eq!(0, unsafe { std::mem::transmute::<Option<Size>, u8>(None) });
+}