Short and Int are different for GCC

Lebeau

« on: 22 / June / 2011, 16:08:38 »

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Well, I just get a look to dump-a650-100d.txt to see difference between -O3 and -Os gcc option.

I was extremely surprise to see how -- short get_metering_mode_for_exif( short...) -- was threat, irrelevant to -O option

Code: [Select]

unsigned short get_metering_mode_for_exif ( short metering_mode )
 {
 static unsigned int meter [ 3 ] = { 5, 3, 2 }; // evaluative, spot, centerweightedaverage
 return ( meter [ metering_mode ] );
 }
;

With -- short get_metering_mode_for_exif(short... --

Code: [Select]

000b6488 <get_metering_mode_for_exif>: 
   b6488: 0402       lsls r2, r0, #16       
   b648a: 4b02       ldr r3, [pc, #8] (b6494 <get_metering_mode_for_exif+0xc>)
   b648c: 1391       asrs r1, r2, #14       
   b648e: 1858       adds r0, r3, r1       
   b6490: 8c00       ldrh r0, [r0, #32]  
   b6492: 4770       bx lr       
   b6494: 000e0ec0  .word 0x000e0ec0

Get attention to lsls and asrs, and ldrh ...

With -- int get_metering_mode_for_exif(int... --

Code: [Select]

000b152c <get_metering_mode_for_exif>:
   b152c: 4b02       ldr r3, [pc, #8] (b1538 <get_metering_mode_for_exif+0xc>)
   b152e: 0081       lsls r1, r0, #2
   b1530: 1858       adds r0, r3, r1
   b1532: 6a00       ldr r0, [r0, #32]
   b1534: 4770       bx lr
   b1536: 46c0       nop   (mov r8, r8)
   b1538: 000d13fc  .word 0x000d13fc

Well I think short shall be replace by int at many place, no ?

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reyalp

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Re: Short and Int are different for GCC

« Reply #1 on: 22 / June / 2011, 22:40:19 »

For simple values (not arrays or structs) int should almost always be more efficient, both in speed and code size. I'm not sure what motivated the use of short in the original code, I suspect some of it was misguided assumption that it was smaller/faster, some of it was because the underlying canon values are actually shorts, and some of it was just copy/paste of either of those.

You do have to be careful, you can't just do s/short/int/ globally, e.g. some propcase values are are actually only 16 bits. You have to be careful with sign extension, and truncation if you pass it back to a canon function that expects a short.

Note that 'long' is also just there to confuse you. That one could (and should IMO) be eliminated since it's functionally identical to int.

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Lebeau

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Re: Short and Int are different for GCC

« Reply #2 on: 23 / June / 2011, 11:13:06 »

I am concentrating to optimize raw, dng and shot_histogram.
I have choice to change "static int meter" into "static short meter" to eliminate type conversion code or change param and return value to int.
I choose param and return value to int.

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reyalp

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Re: Short and Int are different for GCC

« Reply #3 on: 23 / June / 2011, 15:59:39 »

Quote from: Lebeau on 23 / June / 2011, 11:13:06

I am concentrating to optimize raw, dng and shot_histogram.
I have choice to change "static int meter" into "static short meter" to eliminate type conversion code or change param and return value to int.
I choose param and return value to int.

Not sure exactly what these are. If you are using an array of histogram values, I would definitely use the smallest sized value it can fit in.

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Lebeau

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Re: Short and Int are different for GCC

« Reply #4 on: 24 / June / 2011, 07:39:30 »

These short-int combination are in dng.c

I agree, when creating arrays, you shall use the smallest size as possible to limit mem size and optimize cache traffic.

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