[llvm-dev] Early CSE clobbering llvm.assume

[Daniel Berlin via llvm-dev]

So,

On Fri, Jun 10, 2016, 7:33 PM Lawrence, Peter <c_plawre at qca.qualcomm.com>
wrote:

> Daniel,
>
>        Allow me to elaborate some more...
>
>
>
> You seem to be trying to get “Constant-propagation” out of an “assume”,
>


Actually, no. That is mostly what llvm tries to get, not me.  It does get
some range info, but not a lot.

I think it's pretty much designed wrong, and in need of revision to get
good info out of.  Not that I think it was a bad idea to start with
something, but I think we need to revise it.

But a good compiler does so much more
>

Fwiw, gvn already gets all these, but it is very complicated.

>      *  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
>
>
>
>
>
>
>
> *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>
> 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]
> *Sent:* Friday, June 10, 2016 5:39 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
>
>
>
>
>
>
>
> On Fri, Jun 10, 2016 at 5:34 PM, Lawrence, Peter via llvm-dev <
> 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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