bjoernager/rust
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Auto merge of #89030 - nbdd0121:box2, r=jonas-schievink

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Introduce `Rvalue::ShallowInitBox`

Polished version of #88700.

Implements MCP rust-lang/compiler-team#460, and should allow #43596 to go forward.

In short, creating an empty box is split from a nullary-op `NullOp::Box` into two steps, first a call to `exchange_malloc`, then a `Rvalue::ShallowInitBox` which transmutes `*mut u8` to a shallow-initialized `Box<T>`. This allows the `exchange_malloc` call to unwind. Details can be found in the MCP.

`NullOp::Box` is not yet removed, purely to make reverting easier in case anything goes wrong as the result of this PR. If revert is needed a reversion of "Use Rvalue::ShallowInitBox for box expression" commit followed by a test bless should be sufficient.

Experiments in #88700 showed a very slight compile-time perf regression due to (supposedly) slightly more time spent in LLVM. We could omit unwind edge generation (in non-`oom=panic` case) in box expression MIR construction to restore perf; but I don't think it's necessary since runtime perf isn't affected and perf difference is rather small.
This commit is contained in:
bors 2021-09-25 11:01:13 +00:00
commit e9f29a8519
31 changed files with 478 additions and 237 deletions
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13
compiler/rustc_codegen_ssa/src/mir/rvalue.rs
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@ -550,6 +550,18 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
OperandRef::new_zst(&mut bx, self.cx.layout_of(self.monomorphize(ty)));
(bx, operand)
}
mir::Rvalue::ShallowInitBox(ref operand, content_ty) => {
let operand = self.codegen_operand(&mut bx, operand);
let lloperand = operand.immediate();
let content_ty = self.monomorphize(content_ty);
let box_layout = bx.cx().layout_of(bx.tcx().mk_box(content_ty));
let llty_ptr = bx.cx().backend_type(box_layout);
let val = bx.pointercast(lloperand, llty_ptr);
let operand = OperandRef { val: OperandValue::Immediate(val), layout: box_layout };
(bx, operand)
}
}
}
@ -763,6 +775,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
mir::Rvalue::AddressOf(..) |
mir::Rvalue::Len(..) |
mir::Rvalue::Cast(..) | // (*)
mir::Rvalue::ShallowInitBox(..) | // (*)
mir::Rvalue::BinaryOp(..) |
mir::Rvalue::CheckedBinaryOp(..) |
mir::Rvalue::UnaryOp(..) |