Closed
Description
The following pattern occurs reasonably often in the standard lib:
fn new() -> SomeStruct {
let mut foo = SomeStruct { empty... };
// ... initialise foo ...
foo
}
which currently uses a memcpy at the end to copy the result into the out pointer, rather than just directly placing foo into it to begin with.
An example:
pub fn foo() -> [uint, .. 8] {
[0, .. 8]
}
pub fn bar() -> [uint, .. 8] {
let a = [0, .. 8];
a
}Compiling with -O -S --emit-llvm:
define void @_ZN3foo15_5274bbbd7427d03_00E([8 x i64]* nocapture, { i64, %tydesc*, i8*, i8*, i8 } addrspace(1)* nocapture) #1 {
static_allocas:
%2 = bitcast [8 x i64]* %0 to i8*
call void @llvm.memset.p0i8.i64(i8* %2, i8 0, i64 64, i32 8, i1 false)
ret void
}
define void @_ZN3bar15_5274bbbd7427d03_00E([8 x i64]* nocapture, { i64, %tydesc*, i8*, i8*, i8 } addrspace(1)* nocapture) #1 {
static_allocas:
%2 = alloca [8 x i64], align 8
%3 = bitcast [8 x i64]* %2 to i8*
call void @llvm.memset.p0i8.i64(i8* %3, i8 0, i64 64, i32 8, i1 false)
%4 = bitcast [8 x i64]* %0 to i8*
call void @llvm.memcpy.p0i8.p0i8.i64(i8* %4, i8* %3, i64 64, i32 8, i1 false)
ret void
}