[PATCH] D69023: [Alignment][NFC] Optimize alignTo

[Guillaume Chatelet via Phabricator via llvm-commits]

gchatelet created this revision.
gchatelet added reviewers: jakehehrlich, courbet.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

A small optimization suggested by jakehehrlich@ in D64790 <https://reviews.llvm.org/D64790>.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D69023

Files:
  llvm/include/llvm/Support/Alignment.h


Index: llvm/include/llvm/Support/Alignment.h
===================================================================
--- llvm/include/llvm/Support/Alignment.h
+++ llvm/include/llvm/Support/Alignment.h
@@ -161,7 +161,17 @@
 
 /// Returns a multiple of A needed to store `Size` bytes.
 inline uint64_t alignTo(uint64_t Size, Align A) {
-  return (Size + A.value() - 1) / A.value() * A.value();
+  const uint64_t value = A.value();
+  // The following line is equivalent to `(Size + value - 1) / value * value`.
+
+  // The division followed by a multiplication can be thought of as a right
+  // shift followed by a left shift which zeros out the extra bits produced in
+  // the bump; `-value` is a mask where all those bits being zeroed out are just
+  // zero.
+
+  // Most compilers can generate this code but the pattern may be missed when
+  // multiple functions gets inlined.
+  return (Size + value - 1) & -value;
 }
 
 /// Returns a multiple of A needed to store `Size` bytes.


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