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Re: wide int patch #7: Replacement of INT_CST_LT and INT_CST_LT_UNSIGNED


On 10/19/2012 07:13 AM, Richard Biener wrote: 
On Fri, Oct 19, 2012 at 12:59 PM, Kenneth Zadeck
<zadeck@naturalbridge.com> wrote:
On 10/19/2012 04:22 AM, Richard Biener wrote: 
On Thu, Oct 18, 2012 at 7:32 PM, Kenneth Zadeck
<zadeck@naturalbridge.com> wrote:

This patch replaces all instances of  INT_CST_LT and INT_CST_LT_UNSIGNED
with
the new functions tree_int_cst_lts_p and tree_int_cst_ltu_p.   With the
new
implementation of int_cst these functions will be too big to do inline.

These new function names are extremely confusing given that we already
have tree_int_cst_lt which does the right thing based on the signedness
of the INTEGER_CST trees.

The whole point of the macros was to be inlined and you break that.  That
is,
for example

         if (unsignedp && unsignedp0)
          {
-         min_gt = INT_CST_LT_UNSIGNED (primop1, minval);
-         max_gt = INT_CST_LT_UNSIGNED (primop1, maxval);
-         min_lt = INT_CST_LT_UNSIGNED (minval, primop1);
-         max_lt = INT_CST_LT_UNSIGNED (maxval, primop1);
+         min_gt = tree_int_cst_ltu_p (primop1, minval);
+         max_gt = tree_int_cst_ltu_p (primop1, maxval);
+         min_lt = tree_int_cst_ltu_p (minval, primop1);
+         max_lt = tree_int_cst_ltu_p (maxval, primop1);
          }
         else
          {
-         min_gt = INT_CST_LT (primop1, minval);
-         max_gt = INT_CST_LT (primop1, maxval);
-         min_lt = INT_CST_LT (minval, primop1);
-         max_lt = INT_CST_LT (maxval, primop1);
+         min_gt = tree_int_cst_lts_p (primop1, minval);
...

could have just been

      min_gt = tree_int_cst_lt (primop1, minval);
....

without any sign check.

So if you think you need to kill the inlined variants please use the
existing
tree_int_cst_lt instead.

no, they could not have.   tree_int_cst_lt uses the signedness of the type
to determine how to do the comparison.    These two functions, as the macros
they replace, force the comparison to be done independent of the signedness
of the type.

Well, looking at the surrounding code it's indeed non-obvious that this would
be a 1:1 transform.  But then they should not compare _trees_ but instead
compare double-ints (or wide-ints).

That said, I still think having a tree_int_cst_lt[us]_p function is wrong.
tree INTEGER_CSTs have a sign.  (apart from that opinion we have
tree_int_cst_lt and you introduce tree_int_cst_ltu_p - consistent
would be tree_int_cst_ltu).

This reply applies just as much to this patch as patch 6.
I morally agree with you 100%.    But the code does not agree with you.

On patch 6, there are about 450 places where we want to take the lower hwi worth of bits out of a int cst. Of those, only 5 places use the function that allows the signedness to be passed in. The rest make the signedness decision based on the local code and completely ignore any information in the type. Of those 5 that do allow the signedness to be passed in, only three of them actually pass it in based on the signedness of the variable they are accessing.

I am sure that a lot of these are wrong. But i could not tell you which are and which are not.
luckily, a lot of this will go away with the full wide-int code because i just do most of this math in the full precision so the issue never comes up. But after i am finished, there will still be a fair number of places that do this. (luckily, a large number of them are pulling the number out and comparing it to the precision of something, so this is likely to be harmless no matter how the code is written).

But to a large extent, you are shooting the messenger here, and not person who committed the crime. I will be happy to add some comments to point the clients of these to the one that looks at the type. In looking over the patch, the only obvious ones that could be changed are the ones in tree-ssa-uninit.c and the tree-vrp.c. The one in tree-vrp.c just looks like that the person writing the code did not know about tree_int_cst_lt and wrote the check out our himself. (i will fix this in the tree-vrp patch that i am working on now. The one in tree-ssa-uniunit looks correct.

But beyond that, the rest are in the front ends and so i think that this as good as you get out of me.

Kenny

I do not know why we need to do this.  I am just applying a plug compatible
replacement here. I did not write this code, but I do not think that i can
just do as you say here.

So use the double-int interface in the places you substituted your new
tree predicates.  Yes, you'll have to touch that again when converting to
wide-int - but if those places really want to ignore the sign of the tree
they should not use a tree interface.

Richard.

Kenny


Thanks,
Richard.

This is a small patch that has no prerequisites.

Tested on x86-64.

kenny


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