asm_avr.inc: add missing memory clobber#238
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It seems that older versions of GCC did not optimize memory accesses across function calls. That made function calls an implicit memory barrier, even when the function implementation was part of the current compilation unit and the compiler could infer that a function did not touch memory. With inline assembly, the compiler assumes the clobber list to be exhaustive. If no memory clobber is given and a function does not touch memory except for the inline assembly, the compiler may assume that a function does not change memory contents and optimize accordingly. Adding the required memory clobbers fixes incorrect computations of `vli_mmod_fast_secp256r1()` on AVR GCC 15.2.0.
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I wonder if using dummy memory operands would be preferable (ie, provide more accurate constraints to the compiler). It looks like the syntax should be |
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Thx for the pointer, I'll give it a spin :-) |
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It seems that older versions of GCC did not optimize memory accesses across function calls. That made function calls an implicit memory barrier, even when the function implementation was part of the current compilation unit and the compiler could infer that a function did not touch memory.
With inline assembly, the compiler assumes the clobber list to be exhaustive. If no memory clobber is given and a function does not touch memory except for the inline assembly, the compiler may assume that a function does not change memory contents and optimize accordingly.
Adding the required memory clobbers fixes incorrect computations of
vli_mmod_fast_secp256r1()on AVR GCC 15.2.0.Fixes #236