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Auto merge of #79801 - eddyb:scalar-transmute, r=nagisa

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rustc_codegen_ssa: use bitcasts instead of type punning for scalar transmutes.

This specifically helps with `f32` <-> `u32` (`from_bits`, `to_bits`) in Rust-GPU (`rustc_codegen_spirv`), where (AFAIK) we don't yet have enough infrastructure to turn type punning memory accesses into SSA bitcasts.
(There may be more instances, but the one I've seen myself is `f32::signum` from `num-traits` inspecting e.g. the sign bit)

Sadly I've had to make an exception for `transmute`s between pointers and non-pointers, as LLVM disallows using `bitcast` for them.

r? `@nagisa` cc `@khyperia`
This commit is contained in:
bors 2020-12-10 12:55:12 +00:00
commit d7560e8968
2 changed files with 104 additions and 0 deletions
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19
compiler/rustc_codegen_ssa/src/mir/block.rs
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@ -1395,6 +1395,25 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
dst: PlaceRef<'tcx, Bx::Value>,
) {
let src = self.codegen_operand(bx, src);
// Special-case transmutes between scalars as simple bitcasts.
match (&src.layout.abi, &dst.layout.abi) {
(abi::Abi::Scalar(src_scalar), abi::Abi::Scalar(dst_scalar)) => {
// HACK(eddyb) LLVM doesn't like `bitcast`s between pointers and non-pointers.
if (src_scalar.value == abi::Pointer) == (dst_scalar.value == abi::Pointer) {
assert_eq!(src.layout.size, dst.layout.size);
// NOTE(eddyb) the `from_immediate` and `to_immediate_scalar`
// conversions allow handling `bool`s the same as `u8`s.
let src = bx.from_immediate(src.immediate());
let src_as_dst = bx.bitcast(src, bx.backend_type(dst.layout));
Immediate(bx.to_immediate_scalar(src_as_dst, dst_scalar)).store(bx, dst);
return;
}
}
_ => {}
}
let llty = bx.backend_type(src.layout);
let cast_ptr = bx.pointercast(dst.llval, bx.type_ptr_to(llty));
let align = src.layout.align.abi.min(dst.align);