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<div class="moz-cite-prefix">On 09/27/2016 10:05 PM, Chris Lattner
via llvm-dev wrote:<br>
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<div class="">On Sep 27, 2016, at 9:41 AM, Mehdi Amini <<a
moz-do-not-send="true" href="mailto:mehdi.amini@apple.com"
class="">mehdi.amini@apple.com</a>> wrote:</div>
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<div class="">On Sep 25, 2016, at 5:40 PM, Chris
Lattner <<a moz-do-not-send="true"
href="mailto:clattner@apple.com" class="">clattner@apple.com</a>>
wrote:</div>
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<div style="direction: inherit;" class="">The
pointer could only be null if the length is
zero. If the length is zero, you shouldn't be
loading it.</div>
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<div class="">I was thinking about functions that takes
a non-null pointer. For example, assuming you have:</div>
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<div class="">extern void *my_memcpy (void *dest,
const void *src, size_t len) __attribute__((nonnull
(1, 2)));</div>
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<div class="">You can just be calling it with any
StringRef without a null pointer check.</div>
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<div class="">That said it is just a thought I had, I
don’t have any data or actual example to back this up.</div>
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<div>Yeah, I’m not sure where this comes up, and whether memcpy
should take null when the length is zero is also a highly
contentious thing that flares up in debates periodically.</div>
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One place that might be relevant is the existing special handling in
StringRef around memcmp to avoid undefined behavior due to null
pointers. It hints that we've already hit this issue once. <br>
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Defaulting to null instead of "" is also a
microoptimization.</div>
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<div class="">You mean: because initializing to 0 is
faster than a constant?</div>
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<div>Yes, 0 is a “load immediate”, initializing to “” requires
materializing the address of a global.</div>
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<div>-Chris </div>
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