[llvm-dev] Load combine pass

Sanjoy Das via llvm-dev llvm-dev at lists.llvm.org
Wed Sep 28 17:25:02 PDT 2016

Hi David,

David Chisnall via llvm-dev wrote:
 > On 28 Sep 2016, at 16:50, Philip Reames via 
llvm-dev<llvm-dev at lists.llvm.org>  wrote:
 >> At this point, my general view is that widening transformations of 
any kind should be done very late.  Ideally, this is something the 
backend would do, but doing it as a CGP like fixup pass over the IR is 
also reasonable.
 > I’m really glad to see that this is gone in GVN - it will reduce our
 > diffs a lot when we do the next import.  The GVN load widening is not
 > sound in the presence of hardware-enforced spacial memory safety, so
 > we ended up with the compiler inserting things that caused hardware
 > bounds checks to fail and had to disable it in a few places.
 > If you’re reintroducing it, please can we have a backend option to
 > specify whether it’s valid to widen loads beyond the extents (for
 > example, for us it is not valid to widen an i16 load followed by an i8
 > load to an i32 load).  Ideally, we’d also want the back end to supply

Don't you have to mark the functions you generate as
"Attribute::SanitizeAddress"?  We should definitely make the
speculative form of this transform (i.e. "load i32, load i16" -->
"load i64") predicated on Attribute::SanitizeAddress.

-- Sanjoy

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