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The `Box::new(T::default())` implementation of `Box::default` only had two stack copies in debug mode, compared to the current version, which has four. By avoiding creating any `MaybeUninit<T>`'s and just writing `T` directly to the `Box` pointer, the stack usage in debug mode remains the same as the old version.
29 lines
790 B
Rust
29 lines
790 B
Rust
//@ compile-flags: -Copt-level=0
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// Test to make sure that `<Box<T>>::default` does not create too many copies of `T` on the stack.
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// in debug mode. This regressed in dd0620b86721ae8cae86736443acd3f72ba6fc32 to
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// four `T` allocas.
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//
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// See https://github.com/rust-lang/rust/issues/136043 for more context.
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//
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// FIXME: This test only wants to ensure that there are at most two allocas of `T` created, instead
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// of checking for exactly two.
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#![crate_type = "lib"]
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#[allow(dead_code)]
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pub struct Thing([u8; 1000000]);
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impl Default for Thing {
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fn default() -> Self {
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Thing([0; 1000000])
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}
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}
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// CHECK-COUNT-2: %{{.*}} = alloca {{.*}}1000000
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// CHECK-NOT: %{{.*}} = alloca {{.*}}1000000
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#[no_mangle]
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pub fn box_default_single_copy() -> Box<Thing> {
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Box::default()
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}
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