mirror of
https://github.com/c64scene-ar/llvm-6502.git
synced 2025-02-06 23:32:27 +00:00
48fcc07856
ScalarEvolution currently lowers a subtraction recurrence to an add recurrence with the same no-wrap flags as the subtraction. This is incorrect because `sub nsw X, Y` is not the same as `add nsw X, -Y` and `sub nuw X, Y` is not the same as `add nuw X, -Y`. This patch fixes the issue, and adds two test cases demonstrating the bug. Differential Revision: http://reviews.llvm.org/D7081 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226755 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))
//===---------------------------------------------------------------------===//