jit: reuse shared-penalty physregs across preds #9696
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When a block has several Vregs that share the same penalty vector, the per-Vreg penalty tweaks in color_block_initialize don’t enforce a consistent ordering. We can end up with the right physreg set but in a different order than predecessors, so extra edge copies get inserted. After coloring, reorder shared-penalty groups to match the ordering chosen by already-processed predecessors when they agree, keeping the pattern stable and avoiding those redundant copies. This is guarded to AArch64: without xchg, an inconsistent ordering forces a copy2 that lowers to three 4-byte instructions, whereas x86 would emit a single 2-byte xchg.
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When a block has several Vregs that share the same penalty vector, the per-Vreg penalty tweaks in color_block_initialize don’t enforce a consistent ordering. We can end up with the right physreg set but in a different order than predecessors, so extra edge copies get inserted. After coloring, reorder shared-penalty groups to match the ordering chosen by already-processed predecessors when they agree, keeping the pattern stable and avoiding those redundant copies.
This is guarded to AArch64: without xchg, an inconsistent ordering forces a copy2 that lowers to three 4-byte instructions, whereas x86 would emit a single 2-byte xchg.