[llvm-dev] Redundant promotion of integer values in x86 target

[Smith, Kevin B via llvm-dev]

This is an excerpt from Chandler's comment in https://llvm.org/bugs/show_bug.cgi?id=22473


Ø  We need to add a pass that replaces movb (and movw) with movzbl (and movzwl) when the destination is a register and the high bytes aren't used.

I have created such a pass, and have it working in a local workspace of mine.  However, I think that the way it attempts to prove that the high bytes aren't
used is flawed, and so I haven't yet submitted it for community review.

Kevin

From: Sanjay Patel [mailto:spatel at rotateright.com]
Sent: Sunday, January 31, 2016 9:57 AM
To: Taewook Oh <twoh at fb.com>
Cc: llvm-dev at lists.llvm.org; Smith, Kevin B <kevin.b.smith at intel.com>
Subject: Re: [llvm-dev] Redundant promotion of integer values in x86 target

Hi Taewook -
There's a discussion about the underlying x86 micro-arch details here:
http://comments.gmane.org/gmane.comp.compilers.llvm.cvs/167221

The conclusion was that we should change how we currently handle these, but we don't want to regress the case that was addressed by:
http://reviews.llvm.org/rL195496

There are open bugs with more discussion related to this:
https://llvm.org/bugs/show_bug.cgi?id=17113
https://llvm.org/bugs/show_bug.cgi?id=22473
https://llvm.org/bugs/show_bug.cgi?id=22532
https://llvm.org/bugs/show_bug.cgi?id=23155  (cc'ing Kevin in case there's any update on this one)


On Fri, Jan 29, 2016 at 10:04 PM, Taewook Oh via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hello,

While looking at some internal benchmarks, I found that llvm generates codes with redundant promotion, something like:

xor %al, %cl
movzbl %cl, %ecx
cmp $0x20, %ecx

I believe that the promotion stems from the logic in X86TargetLowering::EmitCmp. Comments in the code says,

"Do the comparison at i32 if it's smaller, besides the Atom case. This avoids subregister aliasing issues. Keep the smaller reference if we're optimizing for size, however, as that'll allow better folding of memory operations."

Can anybody please explain me more in detail about the subregister aliasing issues?

Thanks,
Taewook

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