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bors 446649d463 Auto merge of #137513 - scottmcm:identity-transmute, r=saethlin 
Don't re-`assume` in `transmute`s that don't change niches

I noticed in nightly 2025-02-21 that `transmute` is emitting way more `assume`s than necessary for newtypes.

For example, the three transmutes in <https://rust.godbolt.org/z/fW1KaTc4o> emits
```rust
define noundef range(i32 1, 0) i32 `@repeatedly_transparent_transmute(i32` noundef range(i32 1, 0) %_1) unnamed_addr {
start:
  %0 = sub i32 %_1, 1
  %1 = icmp ule i32 %0, -2
  call void `@llvm.assume(i1` %1)
  %2 = sub i32 %_1, 1
  %3 = icmp ule i32 %2, -2
  call void `@llvm.assume(i1` %3)
  %4 = sub i32 %_1, 1
  %5 = icmp ule i32 %4, -2
  call void `@llvm.assume(i1` %5)
  %6 = sub i32 %_1, 1
  %7 = icmp ule i32 %6, -2
  call void `@llvm.assume(i1` %7)
  %8 = sub i32 %_1, 1
  %9 = icmp ule i32 %8, -2
  call void `@llvm.assume(i1` %9)
  %10 = sub i32 %_1, 1
  %11 = icmp ule i32 %10, -2
  call void `@llvm.assume(i1` %11)
  ret i32 %_1
}
```

But those are all just newtypes that don't change size or niches, so none of it's needed.

After this PR it's down to just
```rust
define noundef range(i32 1, 0) i32 `@repeatedly_transparent_transmute(i32` noundef range(i32 1, 0) %_1) unnamed_addr {
start:
  ret i32 %_1
}
```
because none of those `assume`s in the original actually did anything.

(Transmuting to something with a difference niche, though, still has the assumes -- the other tests continue to pass checking that.)
2025-03-09 01:25:48 +00:00
..
src Auto merge of #137513 - scottmcm:identity-transmute, r=saethlin 2025-03-09 01:25:48 +00:00
Cargo.toml compiler: bump cc to 1.2.16 to fix x86 Windows jobs on newest Windows SDK 2025-02-28 21:10:21 +08:00
messages.ftl improve error message and testing of using an unsigned simd mask 2025-03-01 12:58:22 +01:00
README.md mv compiler to compiler/ 2020-08-30 18:45:07 +03:00

README.md

Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.