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fc72ae613a
Typically instcombine has handled this, but pointer differences show up in several contexts where we would like to get constant folding, and cannot afford to run instcombine. Specifically, I'm working on improving the constant folding of arguments used in inline cost analysis with instsimplify. Doing this in instsimplify implies some algorithm changes. We have to handle multiple layers of all-constant GEPs because instsimplify cannot fold them into a single GEP the way instcombine can. Also, we're only interested in all-constant GEPs. The result is that this doesn't really replace the instcombine logic, it's just complimentary and focused on constant folding. Reviewed on IRC by Benjamin Kramer. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@152555 91177308-0d34-0410-b5e6-96231b3b80d8
Analysis Opportunities:
//===---------------------------------------------------------------------===//
In test/Transforms/LoopStrengthReduce/quadradic-exit-value.ll, the
ScalarEvolution expression for %r is this:
{1,+,3,+,2}<loop>
Outside the loop, this could be evaluated simply as (%n * %n), however
ScalarEvolution currently evaluates it as
(-2 + (2 * (trunc i65 (((zext i64 (-2 + %n) to i65) * (zext i64 (-1 + %n) to i65)) /u 2) to i64)) + (3 * %n))
In addition to being much more complicated, it involves i65 arithmetic,
which is very inefficient when expanded into code.
//===---------------------------------------------------------------------===//
In formatValue in test/CodeGen/X86/lsr-delayed-fold.ll,
ScalarEvolution is forming this expression:
((trunc i64 (-1 * %arg5) to i32) + (trunc i64 %arg5 to i32) + (-1 * (trunc i64 undef to i32)))
This could be folded to
(-1 * (trunc i64 undef to i32))
//===---------------------------------------------------------------------===//