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rust/tests/codegen/placement-new.rs

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add initial tests for placement new changes
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//@ compile-flags: -O
add codegen test
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//@ compile-flags: -Zmerge-functions=disabled
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#![crate_type = "lib"]
// Test to check that types with "complex" destructors, but trivial `Default` impls
// are constructed directly into the allocation in `Box::default` and `Arc::default`.
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use std::rc::Rc;
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use std::sync::Arc;
// CHECK-LABEL: @box_default_inplace
#[no_mangle]
pub fn box_default_inplace() -> Box<(String, String)> {
allocate before calling T::default in <Box<T>>::default() The `Box<T: Default>` impl currently calls `T::default()` before allocating the `Box`. Most `Default` impls are trivial, which should in theory allow LLVM to construct `T: Default` directly in the `Box` allocation when calling `<Box<T>>::default()`. However, the allocation may fail, which necessitates calling `T's` destructor if it has one. If the destructor is non-trivial, then LLVM has a hard time proving that it's sound to elide, which makes it construct `T` on the stack first, and then copy it into the allocation. Create an uninit `Box` first, and then write `T::default` into it, so that LLVM now only needs to prove that the `T::default` can't panic, which should be trivial for most `Default` impls.
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// CHECK-NOT: alloca
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// CHECK: [[BOX:%.*]] = {{.*}}call {{.*}}__rust_alloc(
allocate before calling T::default in <Box<T>>::default() The `Box<T: Default>` impl currently calls `T::default()` before allocating the `Box`. Most `Default` impls are trivial, which should in theory allow LLVM to construct `T: Default` directly in the `Box` allocation when calling `<Box<T>>::default()`. However, the allocation may fail, which necessitates calling `T's` destructor if it has one. If the destructor is non-trivial, then LLVM has a hard time proving that it's sound to elide, which makes it construct `T` on the stack first, and then copy it into the allocation. Create an uninit `Box` first, and then write `T::default` into it, so that LLVM now only needs to prove that the `T::default` can't panic, which should be trivial for most `Default` impls.
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// CHECK-NOT: call void @llvm.memcpy
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// CHECK: ret ptr [[BOX]]
Box::default()
}
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// CHECK-LABEL: @rc_default_inplace
#[no_mangle]
pub fn rc_default_inplace() -> Rc<(String, String)> {
// CHECK-NOT: alloca
// CHECK: [[RC:%.*]] = {{.*}}call {{.*}}__rust_alloc(
// CHECK-NOT: call void @llvm.memcpy
// CHECK: ret ptr [[RC]]
Rc::default()
}
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// CHECK-LABEL: @arc_default_inplace
#[no_mangle]
pub fn arc_default_inplace() -> Arc<(String, String)> {
allocate before calling T::default in <Arc<T>>::default() Same rationale as in the previous commit.
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// CHECK-NOT: alloca
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// CHECK: [[ARC:%.*]] = {{.*}}call {{.*}}__rust_alloc(
allocate before calling T::default in <Arc<T>>::default() Same rationale as in the previous commit.
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// CHECK-NOT: call void @llvm.memcpy
add initial tests for placement new changes
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// CHECK: ret ptr [[ARC]]
Arc::default()
}
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