Loop idiom recognition avoids speculating no-op load/store.
GCC's loop idiom recognition avoids speculating load/store pairs for modes that cannot transfer bits, preventing undefined behavior.
The loop idiom recognition pass in GCC was speculating load/store pairs for modes where bit transfer is impossible. This commit prevents the speculation for such modes, including boolean types and bitfield accesses. It also avoids undefined behavior when bitfield loads are out-of-bounds.
In Details
GCC's loop idiom recognition (LIM) pass attempts to identify common loop patterns and transform them into more efficient code. This change addresses a bug where LIM would speculate on memory accesses even when the data type couldn't be meaningfully loaded or stored, or when a bitfield access would result in undefined behavior. The fix involves modifying the execute_sm function in tree-ssa-loop-im.cc to account for these cases.
For Context
Modern compilers use many optimization passes to improve code performance. One such pass is loop idiom recognition, which identifies common patterns in loops and replaces them with faster equivalents. This commit fixes a bug in GCC where the compiler would introduce potentially invalid load/store operations during loop optimization, especially when dealing with bitfields or boolean types. The fix ensures that the compiler only speculates on valid memory transfers, preventing undefined behavior.