[llvm] r375000 - [Alignment][NFC] Optimize alignTo

[Guillaume Chatelet via llvm-commits]

Author: gchatelet
Date: Wed Oct 16 06:06:17 2019
New Revision: 375000

URL: http://llvm.org/viewvc/llvm-project?rev=375000&view=rev
Log:
[Alignment][NFC] Optimize alignTo

Summary: A small optimization suggested by jakehehrlich@ in D64790.

Reviewers: jakehehrlich, courbet

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69023

Modified:
    llvm/trunk/include/llvm/Support/Alignment.h

Modified: llvm/trunk/include/llvm/Support/Alignment.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/Alignment.h?rev=375000&r1=374999&r2=375000&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Support/Alignment.h (original)
+++ llvm/trunk/include/llvm/Support/Alignment.h Wed Oct 16 06:06:17 2019
@@ -161,7 +161,17 @@ inline bool isAddrAligned(Align Lhs, con
 
 /// 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.