Re: [PATCH 1/2] rtlwifi: Fix improve function 'rtl_addr_delay()' in core.c

From: Larry Finger
Date: Wed Feb 03 2016 - 14:44:59 EST

On 02/03/2016 11:49 AM, ByeoungWook Kim wrote:
Hi David,

2016-02-03 23:41 GMT+09:00 David Laight <David.Laight@xxxxxxxxxx>:
From: Byeoungwook Kim
Sent: 03 February 2016 02:00
Conditional codes in rtl_addr_delay() were improved in readability and
performance by using switch codes.
...
void rtl_addr_delay(u32 addr)
{
- if (addr == 0xfe)
+ switch (addr) {
+ case 0xfe:
mdelay(50);
- else if (addr == 0xfd)
+ break;
+ case 0xfd:
mdelay(5);
- else if (addr == 0xfc)
+ break;
+ case 0xfc:
mdelay(1);
- else if (addr == 0xfb)
+ break;
+ case 0xfb:
udelay(50);
- else if (addr == 0xfa)
+ break;
+ case 0xfa:
udelay(5);
- else if (addr == 0xf9)
+ break;
+ case 0xf9:
udelay(1);
+ break;
+ };

Straight 'performance' can't matter here, not with mdelay(50)!
The most likely effect is from speeding up the 'don't delay' path
and reducing the number of conditionals (and hence accuracy of) udelay(1).
Reversing the if-chain might be better still.


I agree with your assists about "The most likely effect is from
speeding up the 'don't delay' path and reducing the number of
conditionals (and hence accuracy of) udelay(1).".

I converted to assembly codes like next line from conditionals.

---

if (addr == 0xf9)
00951445 cmp dword ptr [addr],0F9h
0095144C jne main+35h (0951455h)
a();
0095144E call a (09510EBh)
00951453 jmp main+83h (09514A3h)
else if (addr == 0xfa)
00951455 cmp dword ptr [addr],0FAh
0095145C jne main+45h (0951465h)
a();
0095145E call a (09510EBh)
00951463 jmp main+83h (09514A3h)
else if (addr == 0xfb)
00951465 cmp dword ptr [addr],0FBh
0095146C jne main+55h (0951475h)
a();
0095146E call a (09510EBh)
00951473 jmp main+83h (09514A3h)
else if (addr == 0xfc)
00951475 cmp dword ptr [addr],0FCh
0095147C jne main+65h (0951485h)
b();
0095147E call b (09510E6h)
00951483 jmp main+83h (09514A3h)
else if (addr == 0xfd)
00951485 cmp dword ptr [addr],0FDh
0095148C jne main+75h (0951495h)
b();
0095148E call b (09510E6h)
00951493 jmp main+83h (09514A3h)
else if (addr == 0xfe)
00951495 cmp dword ptr [addr],0FEh
0095149C jne main+83h (09514A3h)
b();
0095149E call b (09510E6h)

---

if the addr value was 0xfe, Big-O-notation is O(1).
but if the addr value was 0xf9, Big-O-notation is O(n).

2016-02-03 23:41 GMT+09:00 David Laight <David.Laight@xxxxxxxxxx>:
From: Byeoungwook Kim
Sent: 03 February 2016 02:00
Conditional codes in rtl_addr_delay() were improved in readability and
performance by using switch codes.

I'd like to see the performance data :-)

I used switch codes to solve of this problem.

If the addr variable was increment consecutive, switch codes is able
to use branch table for optimize.
so I converted to assembly codes like next line from same codes about my patch.

switch (addr)
011C1445 mov eax,dword ptr [addr]
011C1448 mov dword ptr [ebp-0D0h],eax
011C144E mov ecx,dword ptr [ebp-0D0h]
011C1454 sub ecx,0F9h
011C145A mov dword ptr [ebp-0D0h],ecx
011C1460 cmp dword ptr [ebp-0D0h],5
011C1467 ja $LN6+28h (011C149Eh)
011C1469 mov edx,dword ptr [ebp-0D0h]
011C146F jmp dword ptr [edx*4+11C14B4h]
{
case 0xf9: a(); break;
011C1476 call a (011C10EBh)
011C147B jmp $LN6+28h (011C149Eh)
case 0xfa: a(); break;
011C147D call a (011C10EBh)
011C1482 jmp $LN6+28h (011C149Eh)
case 0xfb: a(); break;
011C1484 call a (011C10EBh)
011C1489 jmp $LN6+28h (011C149Eh)
case 0xfc: b(); break;
011C148B call b (011C10E6h)
011C1490 jmp $LN6+28h (011C149Eh)
case 0xfd: b(); break;
011C1492 call b (011C10E6h)
011C1497 jmp $LN6+28h (011C149Eh)
case 0xfe: b(); break;
011C1499 call b (011C10E6h)
}

===[[branch table]]===
011C14B4 011C1476h
011C14B8 011C147Dh
011C14BC 011C1484h
011C14C0 011C148Bh
011C14C4 011C1492h
011C14C8 011C1499h

So conditional codes into rtl_addr_delay() can improve to readability
and performance that used switch codes.

My advice is that you relax. I was out of my office for a day and a half, and I return to find my inbox full of this topic. The discussion is OK, but submitting 3 versions of a patch before I (the maintainer) even have a chance to read the original submission. When resubmitting a new version of a multi-patch set, every member of that set should be resubmitted with the new version even though a particular member has not changed. This convention makes it easier for the maintainer to keep track of the changes. In addition, all patches are listed together in patchwork.

The performance will depend on where you satisfy the condition. All switch cases have the same execution time, but in the if .. else if .. else form, the earlier tests execute more quickly. I'm not sure that one can make any blanket statement about performance. Certainly, the switch version will be larger. For a switch with 8 cases plus default, the object code if 43 bytes larger than the nested ifs in a test program that I created. That is a significant penalty.

I agree that a switch statement would be clearer than the nested ifs for cases where multiple cases used the same code, of if the paragraphs were complicated. As neither situation is involved here, I consider the patch to rtl_addr_delay() to be just a churning of the source. As any change carries a non-zero probability of problems, it is better to make only important changes. In addition, you should respect the style of the original author as long it is not wrong. Thus

NACK

Larry