Assignment in result variables generates genericResets

[edubart]

Assignment to result variables generate calls to genericReset becoming ~5x slower than raw field assignment, the test case:

import times

type Foo = object
  val: int
  s: seq[int]

proc newFoo(): Foo =
  result.val = 1

proc boo(): Foo =
  result = newFoo()

proc boo2(): Foo =
  let a = newFoo()
  result.s = a.s
  result.val = a.val

proc main() =
  block:
    let start = epochTime()
    for i in 0..<1000000000:
      let a = boo()
    echo "boo ", epochTime() - start

  block:
    let start = epochTime()
    for i in 0..<1000000000:
      let a = boo2()
    echo "boo2 ", epochTime() - start

main()

Outputs:

elapsed 14.58154105735779
elapsed 3.642938910873413

As you can see boo2 that assigns each field separately is faster than boo. The problem is that boo generate genericReset calls which causes the slow down:

boo generates

N_NIMCALL(void, boo_9aSl0jzccntfjPB4osXEnHA)(tyObject_Foo_Bsmztv1sSxNkRrfi5afFAg* Result) {
	genericReset((void*)Result, (&NTI_Bsmztv1sSxNkRrfi5afFAg_));
	newFoo_9aSl0jzccntfjPB4osXEnHA_2(Result);
}

boo2 generates:

N_NIMCALL(void, boo2_9aSl0jzccntfjPB4osXEnHA_3)(tyObject_Foo_Bsmztv1sSxNkRrfi5afFAg* Result) {
	tyObject_Foo_Bsmztv1sSxNkRrfi5afFAg a;
	memset((void*)(&a), 0, sizeof(a));
	memset((void*)(&a), 0, sizeof(a));
	newFoo_9aSl0jzccntfjPB4osXEnHA_2((&a));
	genericSeqAssign((&(*Result).s), a.s, (&NTI_qwqHTkRvwhrRyENtudHQ7g_));
	(*Result).val = a.val;
}

In the boo genericReset call could be avoided and a memset called just like boo2

Workaround: Assign each field separately.

Why is this a problem? As my code is full of returns via result assignment, it's also full of genericReset calls and when profiling genericReset does show on perf with high CPU usage.

[edubart]

The code related to the issue

Nim/compiler/cgen.nim

Line 307 in d874639

if loc.storage != OnStack:

genericReset is called for variables not on the stack, but in the above example the object is on the stack

[edubart]

Workaround 2: Use {.noInit.} in function returning values.

[Araq]

We don't know it's on the stack. You could pass a global / heap location to result which is a hidden pointer.

[Araq]

This issue was fixed in 1.0 or earlier.