Use a faster implementation of maxUIntN.

Summary:
On x86-64 with clang 3.8, before:

   mov     edx, 1
   mov     cl, dil
   shl     rdx, cl
   cmp     rdi, 64
   mov     rax, -1
   cmovne  rax, rdx
   ret

after:

  mov     ecx, 64
  sub     ecx, edi
  mov     rax, -1
  shr     rax, cl
  ret

Reviewers: rnk

Subscribers: dylanmckay, mkuper, llvm-commits

Differential Revision: https://reviews.llvm.org/D22440

llvm-svn: 275718

diff --git a/llvm/include/llvm/Support/MathExtras.h b/llvm/include/llvm/Support/MathExtras.h
index b60732a..e9930f6 100644 (file)
--- a/llvm/include/llvm/Support/MathExtras.h
+++ b/llvm/include/llvm/Support/MathExtras.h
@@ -316,10 +316,11 @@ inline bool isShiftedUInt(uint64_t x) {
 inline uint64_t maxUIntN(uint64_t N) {
   assert(N > 0 && N <= 64 && "integer width out of range");
 
-  // uint64_t(1) << 64 is undefined behavior.
-  if (N == 64)
-    return std::numeric_limits<uint64_t>::max();
-  return (UINT64_C(1) << N) - 1;
+  // uint64_t(1) << 64 is undefined behavior, so we can't do
+  //   (uint64_t(1) << N) - 1
+  // without checking first that N != 64.  But this works and doesn't have a
+  // branch.
+  return UINT64_MAX >> (64 - N);
 }
 
 /// Gets the minimum value for a N-bit signed integer.