GVNSink.cpp File Reference (original) (raw)

This pass attempts to sink instructions into successors, reducing static instruction count and enabling if-conversion. More...

This pass attempts to sink instructions into successors, reducing static instruction count and enabling if-conversion.

We use a variant of global value numbering to decide what can be sunk. Consider:

[ a1 = add i32 b, 1 ] [ c1 = add i32 d, 1 ] [ a2 = xor i32 a1, 1 ] [ c2 = xor i32 c1, 1 ] \ / [ e = phi i32 a2, c2 ] [ add i32 e, 4 ]

GVN would number a1 and c1 differently because they compute different results - the VN of an instruction is a function of its opcode and the transitive closure of its operands. This is the key property for hoisting and CSE.

What we want when sinking however is for a numbering that is a function of the uses of an instruction, which allows us to answer the question "if I replace %a1 with %c1, will it contribute in an equivalent way to all successive instructions?". The PostValueTable class in GVN provides this mapping.

Definition in file GVNSink.cpp.