bors
f676c20edd
codegen: Copy to an alloca when the argument is neither by-val nor by-move for indirect pointer. Fixes https://github.com/rust-lang/rust/issues/155241. When a value is passed via an indirect pointer, the value needs to be copied to a new alloca. For x86_64-unknown-linux-gnu, `Thing` is the case: ```rust #[derive(Clone, Copy)] struct Thing(usize, usize, usize); pub fn foo() { let thing = Thing(0, 0, 0); bar(thing); assert_eq!(thing.0, 0); } #[inline(never)] #[unsafe(no_mangle)] pub fn bar(mut thing: Thing) { thing.0 = 1; } ``` Before passing the thing to the bar function, the thing needs to be copied to an alloca that is passed to bar. ```llvm %0 = alloca [24 x i8], align 8 call void @llvm.memcpy.p0.p0.i64(ptr align 8 %0, ptr align 8 %thing, i64 24, i1 false) call void @bar(ptr %0) ``` This patch applies the rule to the untupled arguments as well. ```rust #![feature(fn_traits)] #[derive(Clone, Copy)] struct Thing(usize, usize, usize); #[inline(never)] #[unsafe(no_mangle)] pub fn foo() { let thing = (Thing(0, 0, 0),); (|mut thing: Thing| { thing.0 = 1; }).call(thing); assert_eq!(thing.0.0, 0); } ``` For this case, this patch changes from ```llvm ; call example::foo::{closure#0} call void @_RNCNvCs15qdZVLwHPA_7example3foo0B3_(ptr ..., ptr %thing) ``` to ```llvm %0 = alloca [24 x i8], align 8 call void @llvm.memcpy.p0.p0.i64(ptr align 8 %0, ptr align 8 %thing, i64 24, i1 false) ; call example::foo::{closure#0} call void @_RNCNvCs15qdZVLwHPA_7example3foo0B3_(ptr ..., ptr %0) ``` However, the same rule cannot be applied to tail calls that would be unsound, because the caller's stack frame is overwritten by the callee's stack frame. Fortunately, https://github.com/rust-lang/rust/pull/151143 has already handled the special case. We must not copy again. No copy is needed for by-move arguments, because the argument is passed to the called "in-place". No copy is also needed for by-val arguments, because the attribute implies that a hidden copy of the pointee is made between the caller and the callee. NOTE: The patch has a trick for tail calls that we pass by-move. We can choose to copy an alloca even for by-move arguments, but tail calls require MUST-by-move.
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