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| author | Yicheng Li <yichengli@chromium.org> | 2019-09-05 11:19:32 -0700 |
|---|---|---|
| committer | Commit Bot <commit-bot@chromium.org> | 2019-10-04 20:57:31 +0000 |
| commit | 1a078e86ab6523d90d8ecc975466c446c030aad0 (patch) | |
| tree | 6f7f01e3cdf7d4c0063466b5558e9c21ccfb67cc /common/util.c | |
| parent | 99d0d6e76c6c9e566a664bdb76f2e3c6db221cbd (diff) | |
| download | chrome-ec-1a078e86ab6523d90d8ecc975466c446c030aad0.tar.gz | |
util: Add function to check whether a buffer is trivial (all 0x00 or all 0xff)
This function's execution time depends only on the buffer length but not on
the specific bytes in the buffer.
BRANCH=nocturne
BUG=chromium:927095
TEST=make -j buildall
TEST=timed the execution of bytes_are_trivial() on a long array with
the following contents:
Array 1: 0x01, 0x00, 0x00, 0x00, ..., 0x00, 0x00 (first byte nontrivial)
Array 2: 0x00, 0x00, 0x00, 0x00, ..., 0x00, 0x02 (last byte nontrivial)
Array 3: 0x00, 0x00, ... , 0x00, 0x03, 0x00, ..., (middle byte nontrivial)
Array 4: 0x00, 0x00 , ... (trivial)
(These 4 arrays have the same length.)
Verified that execution on these arrays take similar amount of time,
proportional to the length of the array, specifically:
For 256k bytes, takes 21~40 microseconds
For 128k bytes, takes 10~17 microseconds
For 64k bytes, takes 5~9 microseconds
For 32k bytes, takes 2~5 microseconds
Because the host timer inaccuracy and potential process scheduling
variations, the execution time for arrays 1-4 are sometimes not exactly
the same. To avoid test flakiness, this timing test is not written to
unit tests. But it should prove that bytes_are_trivial() is a constant
time algorithm.
Change-Id: I131748e1a4ee3a3e19a105dba5dc443bb2371d30
Signed-off-by: Yicheng Li <yichengli@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1787870
Diffstat (limited to 'common/util.c')
| -rw-r--r-- | common/util.c | 12 |
1 files changed, 12 insertions, 0 deletions
diff --git a/common/util.c b/common/util.c index b9f11aad27..8d12dc59f4 100644 --- a/common/util.c +++ b/common/util.c @@ -539,6 +539,18 @@ int get_next_bit(uint32_t *mask) return bit; } +bool bytes_are_trivial(const uint8_t *buffer, size_t size) +{ + size_t i; + uint8_t result0 = 0; + uint8_t result1 = 0; + + for (i = 0; i < size; i++) { + result0 |= buffer[i] ^ 0x00; + result1 |= buffer[i] ^ 0xff; + } + return (result0 == 0) || (result1 == 0); +} /****************************************************************************/ /* stateful conditional stuff */ |