Stop turning transmutes into discriminant reads in mir-opt

[scottmcm]

Partially reverts #109612, as after #109993 these aren't actually equivalent any more, and I'm no longer confident this was ever an improvement in the first place.

Having this "simplification" meant that similar-looking code actually did somewhat different things. For example,

pub unsafe fn demo1(x: std::cmp::Ordering) -> u8 {
    std::mem::transmute(x)
}
pub unsafe fn demo2(x: std::cmp::Ordering) -> i8 {
    std::mem::transmute(x)
}

in nightly today is generating https://rust.godbolt.org/z/dPK58zW18

define noundef i8 @_ZN7example5demo117h341ef313673d2ee6E(i8 noundef %x) unnamed_addr #0 {
  %0 = icmp uge i8 %x, -1
  %1 = icmp ule i8 %x, 1
  %2 = or i1 %0, %1
  call void @llvm.assume(i1 %2)
  ret i8 %x
}

define noundef i8 @_ZN7example5demo217h5ad29f361a3f5700E(i8 noundef %0) unnamed_addr #0 {
  %x = alloca i8, align 1
  store i8 %0, ptr %x, align 1
  %1 = load i8, ptr %x, align 1, !range !2, !noundef !3
  ret i8 %1
}

Which feels too different when the original code is essentially identical.

---

Aside: that example is different after optimizations too:

define noundef i8 @_ZN7example5demo117h341ef313673d2ee6E(i8 noundef returned %x) unnamed_addr #0 {
  %0 = add i8 %x, 1
  %1 = icmp ult i8 %0, 3
  tail call void @llvm.assume(i1 %1)
  ret i8 %x
}

define noundef i8 @_ZN7example5demo217h5ad29f361a3f5700E(i8 noundef returned %0) unnamed_addr #1 {
  ret i8 %0
}

so turning the Transmute into a Discriminant was arguably just making things worse, so leaving it alone instead -- and thus having less code in rustc -- seems clearly better.

[rustbot]

r? @cjgillot

(rustbot has picked a reviewer for you, use r? to override)

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[WaffleLapkin]

@bors r+

[bors]

📌 Commit 363c202 has been approved by WaffleLapkin

It is now in the queue for this repository.

[bors]

⌛ Testing commit 363c202 with merge e9e1bbc...

[bors]

☀️ Test successful - checks-actions
Approved by: WaffleLapkin
Pushing e9e1bbc to master...

[rust-timer]

Finished benchmarking commit (e9e1bbc): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.7%	[-4.7%, -4.7%]	1
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 642.109s -> 642.487s (0.06%)