Messages in this thread | | | Date | Wed, 16 Sep 2015 20:49:42 -0500 | From | Jaime Arrocha <> | Subject | Re: First kernel patch (optimization) |
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On 09/16/2015 07:56 AM, David Laight wrote: > From: Austin S Hemmelgarn >> Sent: 16 September 2015 12:46 >> On 2015-09-15 20:09, Steve Calfee wrote: >>> On Tue, Sep 15, 2015 at 12:53 PM, Eric Curtin <ericcurtin17@gmail.com> wrote: >>>> Signed-off-by: Eric Curtin <ericcurtin17@gmail.com> >>>> >>>> diff --git a/tools/usb/usbip/src/usbip_detach.c b/tools/usb/usbip/src/usbip_detach.c >>>> index 05c6d15..9db9d21 100644 >>>> --- a/tools/usb/usbip/src/usbip_detach.c >>>> +++ b/tools/usb/usbip/src/usbip_detach.c >>>> @@ -47,7 +47,9 @@ static int detach_port(char *port) >>>> uint8_t portnum; >>>> char path[PATH_MAX+1]; >>>> >>>> - >>>> + unsigned int port_len = strlen(port); >>>> + >>>> + for (unsigned int i = 0; i < port_len; i++) >>>> if (!isdigit(port[i])) { >>>> err("invalid port %s", port); >>>> return -1; >>>> >>>> -- >>> Hi Eric, >>> >>> This is fine, but what kind of wimpy compiler optimizer will not move >>> the constant initializer out of the loop? I bet if you compare binary >>> sizes/code it will be exactly the same, and you added some characters >>> of code. Reorganizing code for readability is fine, but for compiler >>> (in)efficiency seems like a bad idea. >> While I agree with your argument, I would like to point out that it is a >> well established fact that GCC's optimizers are kind of brain-dead at >> times and need their hands held. >> >> I'd be willing to bet that the code will be marginally larger (because >> of adding another variable), but might run slightly faster too (because >> in my experience, GCC doesn't always catch things like this), and should >> compile a little faster (because the optimizers don't have to do as much >> work). > The compiler probably can't optimise the strlen(). > If isdigit() is a real function (the locale specific one probably is) > then the compile cannot assume that port[n] isn't changed by the call > to isdigit. > > A simpler change would be: > for (unsigned int i = 0; port[i] != 0; i++) > > Much better would be to use strtoul() instead of atoi(). > > David > I actually took some time to verify this. GCC makes this optimization with -O2 at least on gcc 4.7.2. One interesting observation I found was that in O0 and O2, it does make a call to strlen while in O1 it calculates the length of the string using:
repnz scas %es:(%rdi),%al not %rcx sub $0x2,%rcx
Why does it do that? Is the code above faster? If yes, why not do it in O2 too? Is this still a topic for this forum?
gcc version 4.7.2 (Debian 4.7.2-5) code
void conv_input(char *port) { int portnum;
for(int i = 0; i <strlen(port); i++) if(!isdigit(port[i])) { printf("invalid port %s", port); exit (1); }
portnum = atoi(port); printf("Port number: %d\n", portnum); }
Optimization done? O0 O1 O2 x86 No No Yes amd64 No No Yes
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