[llvm-dev] Early CSE clobbering llvm.assume

[Lawrence, Peter via llvm-dev]

Daniel,
       Allow me to elaborate some more...

You seem to be trying to get “Constant-propagation” out of an “assume”,
But a good compiler does so much more
     *  if(A > B),      assert(A > B),      assume(A > B),     should all result in same optimizations
     *  if(A>B && A<C), assert(A>B && A<C), assume(A>B & A<C), should all result in same optimizations
     *  if(A & 0x1),    assert(A & 0x1),    assume(A & 0x1),   should all result in same optimizations
     *  if(A & (A-1)),  assert(A & (A-1)),  assume(A & (A-1)), should all result in same optimizations

OK, maybe the last one, or two, are a stretch, maybe we only want to generalize to
Value-propagation, and Range-propagation, and not arbitrary Property-propagation,

But this should be the goal,
The question is, is an “intrinsic” the right way to go about it.


--Peter Lawrence.



From: Daniel Berlin [mailto:dberlin at dberlin.org]
Sent: Friday, June 10, 2016 6:32 PM
To: Lawrence, Peter <c_plawre at qca.qualcomm.com>
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Early CSE clobbering llvm.assume

We do, implicitly, because assert generates if conditions. Or at least, gvn knows how to propagate that implicit info. We can do better by exposing it more, most likely
On Fri, Jun 10, 2016, 5:59 PM Lawrence, Peter <c_plawre at qca.qualcomm.com<mailto:c_plawre at qca.qualcomm.com>> wrote:
Daniel,
             Well then my next (dumb?) question is why aren’t we using source level assert information
For optimization ?

--Peter Lawrence.


From: Daniel Berlin [mailto:dberlin at dberlin.org<mailto:dberlin at dberlin.org>]
Sent: Friday, June 10, 2016 5:39 PM
To: Lawrence, Peter <c_plawre at qca.qualcomm.com<mailto:c_plawre at qca.qualcomm.com>>
Cc: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Early CSE clobbering llvm.assume



On Fri, Jun 10, 2016 at 5:34 PM, Lawrence, Peter via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
My (dumb?) question would be:  why is llvm.assume being handled any differently than llvm.assert ?

There is no llvm.assert intrinsic, so i'm not sure what you mean here. Care to give an example?

Other than one trapping and one not-trapping, they should be identical, in both cases they are giving
The optimizers information,  and that shouldn’t be any different from being inside an “if” statement with the same condition ?


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