Fix performance trace inline asm in os_common.h #492
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This commit fixes an inline assembly block that uses `rdtsc` instructin in order to trace the SQLite performance. The issue is that the `__asm__` block is not marked as `__volatile__` and so an optimizing compiler (e.g. GCC 10.1 with -O3 compilation flag) may optimize out a second call to the `hwtime()` function, assuming it should return the same value. This behavior is also described in GCC docs in https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile after the following block: > The following example demonstrates a case where you need to use the volatile qualifier. It uses the x86 rdtsc instruction, which reads the computer’s time-stamp counter. Without the volatile qualifier, the optimizers might assume that the asm block will always return the same value and therefore optimize away the second call. The issue can also bee seen on https://godbolt.org/z/v_a-Qy
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btw this code comes from sqlite and it seems newer sqlite has it fixed. |
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This commit fixes an inline assembly block that uses
rdtscinstruction in order to trace the SQLite performance.The issue is that the
__asm__block is not marked as__volatile__and so an optimizing compiler (e.g. GCC 10.1 with -O3 compilation flag) may optimize out a second call to thehwtime()function, assuming it should return the same value.This behavior is also described in GCC docs in https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile after the following block:
The issue can also bee seen on https://godbolt.org/z/v_a-Qy