Inefficient codegen for function returning String

[jrmuizel]

The following code seems generate inefficient code compared to the C++ equivalent:

pub fn goo() -> String {
    let url_prefix = "https://".to_string();
    url_prefix
}

gives:

define void @_ZN8rust_out3goo17h4b78ef91328cd5a8E(%"3.std::string::String"* noalias nocapture sret dereferenceable(24)) unnamed_addr #0 personality i32 (i32, i32, i64, %"8.unwind::libunwind::_Unwind_Exception"*, %"8.unwind::libunwind::_Unwind_Context"*)* @rust_eh_personality {
_ZN19std..string..String9drop.734817h847d71ab7fd1dc50E.exit:
  %url_prefix = alloca %"3.std::string::String", align 8
  %1 = bitcast %"3.std::string::String"* %url_prefix to i8*
  call void @llvm.lifetime.start(i64 24, i8* %1)
  call void @"_ZN80_$LT$string..String$u20$as$u20$core..convert..From$LT$$RF$$u27$a$u20$str$GT$$GT$4from17hfac1f2dc216f571bE"(%"3.std::string::String"* noalias nocapture nonnull sret dereferenceable(24) %url_prefix, i8* noalias nonnull readonly getelementptr inbounds ([8 x i8], [8 x i8]* @str7345, i64 0, i64 0), i64 8)
  %2 = bitcast %"3.std::string::String"* %0 to i8*
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %2, i8* %1, i64 24, i32 8, i1 false)
  call void @llvm.lifetime.end(i64 24, i8* %1)
  ret void
}

vs.

define void @_Z3goov(%"class.std::__1::basic_string"* noalias sret %agg.result) #0 {
  tail call void @_ZNSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEE6__initEPKcm(%"class.std::__1::basic_string"* %agg.result, i8* getelementptr inbounds ([56 x i8]* @.str, i64 0, i64 0), i64 55)
  ret void
}

The rust code has an additional alloca and a memcpy

[jrmuizel]

pub fn goo() -> String {
"https://".to_string()
}

Gives:
define void @_ZN8rust_out3goo17h4b78ef91328cd5a8E(%"3.std::string::String"* noalias nocapture sret dereferenceable(24)) unnamed_addr #0 {
entry-block:
tail call void @"ZN80$LT$string..String$u20$as$u20$core..convert..From$LT$$RF$$u27$a$u20$str$GT$$GT$4from17hfac1f2dc216f571bE"(%"3.std::string::String"* noalias nocapture nonnull sret dereferenceable(24) %0, i8* noalias nonnull readonly getelementptr inbounds ([8 x i8], [8 x i8]* @str7329, i64 0, i64 0), i64 8)
ret void
}

so it looks like this is just rust-lang/rfcs#788

[jrmuizel]

Perhaps #32966 is more appropriate dup of this.

[jrmuizel]

This is indeed fixed with panic=abort in nightly. I suspect it was fixed by #44992

[jrmuizel]

I filed #45499 about the missing tail calls.