Remove support for the icmp and fcmp constant expressions.
This is part of:
https://discourse.llvm.org/t/rfc-remove-most-constant-expressions/63179
As usual, many of the updated tests will no longer test what they were
originally intended to -- this is hard to preserve when constant
expressions get removed, and in many cases just impossible as the
existence of a specific kind of constant expression was the cause of the
issue in the first place.
The old version of this code would blindly perform arithmetic without
paying attention to whether the types involved were pointers or
integers. This could lead to weird expressions like negating a pointer.
Explicitly handle simple cases involving pointers, like "x < y ? x : y".
In all other cases, coerce the operands of the comparison to integer
types. This avoids the weird cases, while handling most of the
interesting cases.
Differential Revision: https://reviews.llvm.org/D103660
Summary:
Currently, `rewriteLoopExitValues()`'s logic is roughly as following:
> Loop over each incoming value in each PHI node.
> Query whether the SCEV for that incoming value is high-cost.
> Expand the SCEV.
> Perform sanity check (`isValidRewrite()`, D51582)
> Record the info
> Afterwards, see if we can drop the loop given replacements.
> Maybe perform replacements.
The problem is that we interleave SCEV cost checking and expansion.
This is A Problem, because `isHighCostExpansion()` takes special care
to not bill for the expansions that were already expanded, and we can reuse.
While it makes sense in general - if we know that we will expand some SCEV,
all the other SCEV's costs should account for that, which might cause
some of them to become non-high-cost too, and cause chain reaction.
But that isn't what we are doing here. We expand *all* SCEV's, unconditionally.
So every next SCEV's cost will be affected by the already-performed expansions
for previous SCEV's. Even if we are not planning on keeping
some of the expansions we performed.
Worse yet, this current "bonus" depends on the exact PHI node
incoming value processing order. This is completely wrong.
As an example of an issue, see @dmajor's `pr45835.ll` - if we happen to have
a PHI node with two(!) identical high-cost incoming values for the same basic blocks,
we would decide first time around that it is high-cost, expand it,
and immediately decide that it is not high-cost because we have an expansion
that we could reuse (because we expanded it right before, temporarily),
and replace the second incoming value but not the first one;
thus resulting in a broken PHI.
What we instead should do for now, is not perform any expansions
until after we've queried all the costs.
Later, in particular after `isValidRewrite()` is an assertion (D51582)
we could improve upon that, but in a more coherent fashion.
See [[ https://bugs.llvm.org/show_bug.cgi?id=45835 | PR45835 ]]
Reviewers: dmajor, reames, mkazantsev, fhahn, efriedma
Reviewed By: dmajor, mkazantsev
Subscribers: smeenai, nikic, hiraditya, javed.absar, llvm-commits, dmajor
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D79787
