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e9e1bbc7a8
Stop turning transmutes into discriminant reads in mir-opt 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, ```rust 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> ```llvm 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: ```llvm 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. |
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