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Bug 895792 - Fix RoundUpPow2's required precondition to not be wrong. r=terrence

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--HG--
extra : rebase_source : e42420628dad8b8e9fe37287fa8e08eb262fe615
This commit is contained in:
Jeff Walden 2013-07-25 20:01:45 -07:00
parent 98971b1513
commit c184cc6855
2 changed files with 35 additions and 3 deletions
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7
mfbt/MathAlgorithms.h
View File

@ -414,13 +414,14 @@ FloorLog2Size(size_t n)
}
/*
* Round x up to the nearest power of 2. This function assumes that the most
* significant bit of x is not set, which would lead to overflow.
* Compute the smallest power of 2 greater than or equal to |x|. |x| must not
* be so great that the computed value would overflow |size_t|.
*/
inline size_t
RoundUpPow2(size_t x)
{
MOZ_ASSERT(~x > x, "can't round up -- will overflow!");
MOZ_ASSERT(x <= (size_t(1) << (sizeof(size_t) * CHAR_BIT - 1)),
"can't round up -- will overflow!");
return size_t(1) << CeilingLog2(x);
}

31
mfbt/tests/TestCeilingFloor.cpp
View File

@ -7,6 +7,7 @@
using mozilla::CeilingLog2;
using mozilla::FloorLog2;
using mozilla::RoundUpPow2;
static void
TestCeiling()
@ -46,9 +47,39 @@ TestFloor()
MOZ_ASSERT(FloorLog2(i) == 4);
}
static void
TestRoundUpPow2()
{
MOZ_ASSERT(RoundUpPow2(0) == 1);
MOZ_ASSERT(RoundUpPow2(1) == 1);
MOZ_ASSERT(RoundUpPow2(2) == 2);
MOZ_ASSERT(RoundUpPow2(3) == 4);
MOZ_ASSERT(RoundUpPow2(4) == 4);
MOZ_ASSERT(RoundUpPow2(5) == 8);
MOZ_ASSERT(RoundUpPow2(6) == 8);
MOZ_ASSERT(RoundUpPow2(7) == 8);
MOZ_ASSERT(RoundUpPow2(8) == 8);
MOZ_ASSERT(RoundUpPow2(9) == 16);
MOZ_ASSERT(RoundUpPow2(15) == 16);
MOZ_ASSERT(RoundUpPow2(16) == 16);
MOZ_ASSERT(RoundUpPow2(17) == 32);
MOZ_ASSERT(RoundUpPow2(31) == 32);
MOZ_ASSERT(RoundUpPow2(32) == 32);
MOZ_ASSERT(RoundUpPow2(33) == 64);
size_t MaxPow2 = size_t(1) << (sizeof(size_t) * CHAR_BIT - 1);
MOZ_ASSERT(RoundUpPow2(MaxPow2 - 1) == MaxPow2);
MOZ_ASSERT(RoundUpPow2(MaxPow2) == MaxPow2);
// not valid to round up when past the max power of two
}
int main()
{
TestCeiling();
TestFloor();
TestRoundUpPow2();
return 0;
}