llvm-project

8d3f497e - [StructurizeCFG] Hoist and simplify zero-cost incoming else phi values (#139605)

[StructurizeCFG] Hoist and simplify zero-cost incoming else phi values (#139605) The order of if and else blocks can introduce unnecessary VGPR copies. Consider the case of an if-else block where the incoming phi from the 'Else block' only contains zero-cost instructions, and the 'Then' block modifies some value. There would be no interference when coalescing because only one value is live at any point before structurization. However, in the structurized CFG, the Then value is live at 'Else' block due to the path if→flow→else, leading to additional VGPR copies. This patch addresses the issue by: - Identifying PHI nodes with zero-cost incoming values from the Else block and hoisting those values to the nearest common dominator of the Then and Else blocks. - Updating Flow PHI nodes by replacing poison entries (on the if→flow edge) with the correct hoisted values.