Update mmu_get... and mmu_set... by mhightower83 · Pull Request #8290 · esp8266/Arduino (original) (raw)

These changes are needed to address bugs that can emerge with the improved optimization from the GCC 10.3 compiler.

Updated performance inline functions mmu_get_uint8(), ... and mmu_set_uint8(), ... to comply with strict-aliasing rules.
Without this change, stale data may be referenced. This issue was revealed in discussions on #8261 (comment)

Changes to avoid over-optimization of 32-bit wide transfers from IRAM, turning into 8-bit or 16-bit transfers by the new GCC 10.3 compiler. This has been a reoccurring/tricky problem for me with the new compiler.

So far referencing the 32-bit value loaded by way of an Extended ASM R/W output register has stopped the compiler from optimizing down to an 8-bit or 16-bit transfer.

Example:

uint32_t val; __builtin_memcpy(&val, v32, sizeof(uint32_t)); asm volatile ("" :"+r"(val)); // inject 32-bit dependency ...

Updated example irammem.ino

• do a simple test of compliance to strict-aliasing rules
• For mmu_get_uint8(), added tests to evaluate if 32-bit wide transfers were converted to an 8-bit transfer.

Edited: Added missing references and clarifications.