__builtin_memcpy for large blocks - #596
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xavierleroy wants to merge 9 commits into
Open
xavierleroy wants to merge 9 commits into
xavierleroy wants to merge 9 commits into
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Previously, a 32-bit move immediate was used to load `sz / 4`, resulting in an overflow if `sz` >= 2^34. Easy fix: use a 64-bit move immediate. TargetPrinter will turn it into a 32-bit move immediate when possible.
On modern x86_64 processors, it's specially optimized in hardware. Keep the previous sequence (`rep movsl` plus corrections) for x86_32 because it's probably faster on older processors.
Use a 64-bit move immediate for `sz / 16` and do the computations in Z, not int.
Use a 64-bit move immediate and do the calculations in Z, not in int. Note: on RV-32, `sz` is at most 2^32-1, and the 64-bit move immediate only uses RV-32 instructions.
Do the size calculations in Z, not in int.
Do the size calculations in Z, not in int
Larger alignments are not allowed by the formal CompCert semantics.
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The expansion of
__builtin_memcpycurrently masks some of the high bits of the size argument off, resulting in partial copies for very large blocks (size >= 2^30, 2^34 or 2^36, depending on the target architecture).This is very unlikely to occur in practice, but this PR fixes this problem, making sure all bits of the size argument are honored.
In passing, it also changes the expansion of
__builtin_memcpyon x86-64 to take advantage of hardware optimizations on recent x86 processors, and reduces the alignment used to at most 8, which is the largest alignment supported by the CompCert back-end.Issue (on x86-64) reported by Christos Papakonstantinou (Cantina Security).