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Fix undefined behaviour with unaligned loads in syzygy code

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Casting a pointer to a different type with stricter alignment
requirements yields to implementation dependent behaviour.
Practicaly everything is fine for common platforms because the
CPU/OS/compiler will generate correct code, but anyhow it is
better to be safe than sorry.

Testing with dbg_hit_on() shows that the unalignment accesses are
very rare (below 0.1%) so it makes sense to split the code in a
fast path for the common case and a slower path as a fallback.

No functional change (verified with TB enabled).
This commit is contained in:
Joost VandeVondele 2016-11-05 09:04:51 +01:00 • committed by Marco Costalba
parent f5d3f0ded6
commit 876f07cbee
1 changed files with 7 additions and 1 deletions
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8
src/syzygy/tbprobe.cpp
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@ -239,7 +239,13 @@ template<typename T, int LE> T number(void* addr)
const union { uint32_t i; char c[4]; } Le = { 0x01020304 };
const bool IsLittleEndian = (Le.c[0] == 4);
T v = *((T*)addr);
T v;
if ((uintptr_t)addr & (alignof(T) - 1)) // Unaligned pointer (very rare)
std::memcpy(&v, addr, sizeof(T));
else
v = *((T*)addr);
if (LE != IsLittleEndian)
swap_byte(v);
return v;