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am d518a6d3: Merge "Fix the 16 bit/32 bit instruction check for arm."

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* commit 'd518a6d3bccc82cffb61fb7615b8ba09a564dec0':
  Fix the 16 bit/32 bit instruction check for arm.
This commit is contained in:
Christopher Ferris 2015-08-26 19:59:06 +00:00 committed by Android Git Automerger
commit 6501864af4
1 changed files with 16 additions and 9 deletions
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25
libc/bionic/debug_stacktrace.cpp
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@ -90,15 +90,22 @@ static _Unwind_Reason_Code trace_function(__unwind_context* context, void* arg)
// Modify the pc to point at the real function.
if (ip != 0) {
#if defined(__arm__)
// We need to do a quick check here to find out if the previous
// instruction is a Thumb-mode BLX(2). If so subtract 2 otherwise
// 4 from PC.
short* ptr = reinterpret_cast<short*>(ip);
// Thumb BLX(2)
if ((*(ptr-1) & 0xff80) == 0x4780) {
ip -= 2;
} else {
ip -= 4;
// If the ip is suspiciously low, do nothing to avoid a segfault trying
// to access this memory.
if (ip >= 4096) {
// Check bits [15:11] of the first halfword assuming the instruction
// is 32 bits long. If the bits are any of these values, then our
// assumption was correct:
// b11101
// b11110
// b11111
// Otherwise, this is a 16 bit instruction.
uint16_t value = (*reinterpret_cast<uint16_t*>(ip - 2)) >> 11;
if (value == 0x1f || value == 0x1e || value == 0x1d) {
ip -= 4;
} else {
ip -= 2;
}
}
#elif defined(__aarch64__)
// All instructions are 4 bytes long, skip back one instruction.