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authorLei Zhang <thestig@chromium.org>2018-05-23 23:24:50 +0000
committerChromium commit bot <commit-bot@chromium.org>2018-05-23 23:24:50 +0000
commit105ea1c0f896a4a2718f1356cae0dce8c01859cc (patch)
tree9fe951d685244c724da3dbb61761147597ebb504 /core
parent9425006330d98c838cccc5bdb2f5fb11eb204baa (diff)
downloadpdfium-105ea1c0f896a4a2718f1356cae0dce8c01859cc.tar.xz
Avoid repeated calculations in CFX_BitStream::GetBits().
Change-Id: Icfb7c6933625436b9fcf6a9fdfd0e5f655108eca Reviewed-on: https://pdfium-review.googlesource.com/32851 Reviewed-by: dsinclair <dsinclair@chromium.org> Commit-Queue: Lei Zhang <thestig@chromium.org>
Diffstat (limited to 'core')
-rw-r--r--core/fxcrt/cfx_bitstream.cpp21
1 files changed, 13 insertions, 8 deletions
diff --git a/core/fxcrt/cfx_bitstream.cpp b/core/fxcrt/cfx_bitstream.cpp
index 114b5a9d2a..3d3040d96a 100644
--- a/core/fxcrt/cfx_bitstream.cpp
+++ b/core/fxcrt/cfx_bitstream.cpp
@@ -22,29 +22,34 @@ void CFX_BitStream::ByteAlign() {
}
uint32_t CFX_BitStream::GetBits(uint32_t nBits) {
- if (nBits > m_BitSize || m_BitPos + nBits > m_BitSize)
+ ASSERT(nBits > 0);
+ ASSERT(nBits <= 32);
+ if (nBits > m_BitSize || m_BitPos > m_BitSize - nBits)
return 0;
+ const uint32_t bit_pos = m_BitPos % 8;
+ uint32_t byte_pos = m_BitPos / 8;
const uint8_t* data = m_pData.Get();
+ uint8_t current_byte = data[byte_pos];
if (nBits == 1) {
- int bit = (data[m_BitPos / 8] & (1 << (7 - m_BitPos % 8))) ? 1 : 0;
+ int bit = (current_byte & (1 << (7 - bit_pos))) ? 1 : 0;
m_BitPos++;
return bit;
}
- uint32_t byte_pos = m_BitPos / 8;
- uint32_t bit_pos = m_BitPos % 8;
uint32_t bit_left = nBits;
uint32_t result = 0;
if (bit_pos) {
- if (8 - bit_pos >= bit_left) {
- result = (data[byte_pos] & (0xff >> bit_pos)) >> (8 - bit_pos - bit_left);
+ uint32_t bits_readable = 8 - bit_pos;
+ if (bits_readable >= bit_left) {
+ result = (current_byte & (0xff >> bit_pos)) >> (bits_readable - bit_left);
m_BitPos += bit_left;
return result;
}
- bit_left -= 8 - bit_pos;
- result = (data[byte_pos++] & ((1 << (8 - bit_pos)) - 1)) << bit_left;
+ bit_left -= bits_readable;
+ result = (current_byte & ((1 << bits_readable) - 1)) << bit_left;
+ ++byte_pos;
}
while (bit_left >= 8) {
bit_left -= 8;