Qualcomm Insider: Apple 64-Bit Chip `Hit Us in the Gut'

Re:Qualcomm Insider: Apple 64-Bit Chip `Hit Us in the Gut' by data-cat on Monday December 16, 2013 @06:40PM (#6918678)

Wouldn't a 64 bit processor be able to perform more accurate floating point operations faster? I definitely don't believe the only benefit to a 64 bit architecture is being able to address more memory.

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Re:Qualcomm Insider: Apple 64-Bit Chip `Hit Us in the Gut' by colanderman on Monday December 16, 2013 @06:57PM (#6918763)

Depends highly on the architecture. Both floating-point and vector operations are often special-cased in the pipeline (e.g. x86), so e.g. 64-bit floating point operations on a particular 32-bit processor may not exhibit worse performance than if that processor had 64-bit registers.

I'm not familiar enough with the particulars of ARM to answer confidently for floating point operations, but to take an example that's not usually special-cased, say bit vector arithmetic, yes, those operations will execute twice as quickly if they are vectored.

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Re:Qualcomm Insider: Apple 64-Bit Chip `Hit Us in the Gut' by hrydgard on Monday December 16, 2013 @07:10PM (#6918840)

ARM 32-bit did not have SIMD (vector) double precision, while ARM 64-bit does, so here it's definitely a win.

On x86 though, both 32-bit and 64-bit did double precision vectors just fine, so it didn't really apply there (except that the fp register count was doubled).

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Re:Qualcomm Insider: Apple 64-Bit Chip `Hit Us in the Gut' by dragontamer on Monday December 16, 2013 @07:13PM (#6918850)

Only if your code were SIMD aligned. But that is not code that your typical compiler outputs.

Most SIMD code is heavy number-crunching stuff like multimedia or GPU shaders. But much of that low-level handling is handled off CPU on phone platforms. It is simply more power efficient to have a hardware decoder of multimedia.

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Re:Qualcomm Insider: Apple 64-Bit Chip `Hit Us in the Gut' by colanderman on Monday December 16, 2013 @07:26PM (#6918899)

Only if your code were SIMD aligned. But that is not code that your typical compiler outputs.

GCC has supported autovectorization for a while now.

"Unoptimized code will be slow" isn't a great argument anyway. There's not much a processor can do to help that.

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Re:Qualcomm Insider: Apple 64-Bit Chip `Hit Us in the Gut' by dragontamer on Monday December 16, 2013 @07:37PM (#6918936)

Only with proper pragmas and loop constructs.

Besides, most ARM chips supported vectorized code anyway. You know, NEON? http://www.arm.com/products/processors/technologies/neon.php

ARM64 does NOT grant you the vectorized instruction advantage. Qualcomm Snapdragon Krait have supported NEON for some time already.

http://www.anandtech.com/show/5559/qualcomm-snapdragon-s4-kr...

So many people are arguing here, but clearly few of you people have even worked with ARM chips at the assembly level.

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Re:Qualcomm Insider: Apple 64-Bit Chip `Hit Us in the Gut' by colanderman on Monday December 16, 2013 @07:53PM (#6918985)

Only with proper pragmas and loop constructs.

(1) it's enabled by default at -O3; (2) the loop constructs needed are fairly simple; (3) arguing that unoptimized code will be slow is still a poor argument.

ARM64 does NOT grant you the vectorized instruction advantage.

32-bit ARM NEON does not support vectorized doubles. 64-bit ARM NEON does. Source: http://en.wikipedia.org/wiki/ARM_NEON#Advanced_SIMD_.28NEON....

So many people are arguing here, but clearly few of you people have even worked with ARM chips at the assembly level.

Yep. Thankfully I can back up my arguments with quoted facts.

EDIT: and I already granted that vectorized and floating-point operations don't necessarily benefit from larger register widths, so I don't know why you're even arguing. Let alone the OP wasn't even asking specifically about ARM!

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