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

[Guillaume Chatelet via Phabricator via llvm-commits]

This revision was automatically updated to reflect the committed changes.
Closed by commit rG2f6da767f13b: [Alignment][NFC] Optimize alignTo (authored by gchatelet).

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D69023/new/

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 - 1)` 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 - 1);
 }
 
 /// Returns a multiple of A needed to store `Size` bytes.


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D69023.225204.patch
Type: text/x-patch
Size: 993 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20191016/e7112e7c/attachment.bin>