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Omit non-needs_drop drop_in_place in vtables

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This replaces the drop_in_place reference with null in vtables. On
librustc_driver.so, this drops about ~17k dynamic relocations from the
output, since many vtables can now be placed in read-only memory, rather
than having a relocated pointer included.

This makes a tradeoff by adding a null check at vtable call sites.
That's hard to avoid without changing the vtable format (e.g., to use a
pc-relative relocation instead of an absolute address, and avoid the
dynamic relocation that way). But it seems likely that the check is
cheap at runtime.
This commit is contained in:
Mark Rousskov 2024-03-17 17:42:37 -04:00
parent b0f8618938
commit 4c002fce9d
7 changed files with 203 additions and 116 deletions
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5
compiler/rustc_codegen_cranelift/src/base.rs
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@ -548,10 +548,7 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
}
TerminatorKind::Drop { place, target, unwind: _, replace: _ } => {
let drop_place = codegen_place(fx, *place);
crate::abi::codegen_drop(fx, source_info, drop_place);
let target_block = fx.get_block(*target);
fx.bcx.ins().jump(target_block, &[]);
crate::abi::codegen_drop(fx, source_info, drop_place, *target);
}
};
}