Change Details

When implementing memset's today we often see this pattern: $x0 = MOV 0xXYXYXYXYXYXYXYXY store $x0, ... $w1 = MOV 0xXYXYXYXY store $w1, ... We first create a 64bit constant in a 64bit register with all bytes the same and then create a 32bit constant with all bytes the same in a 32bit register. In many targets we could just access the lower byte of the 64bit register instead. - Ideally this would be handled by the ConstantHoist pass but it runs too early when memset isn't expanded yet. - The memset expansion code already had this optimization implemented, however it didn't work as DAGCombiner would fold "trunc(bigconstant)" back to "smallconstant". - This patch makes the memset expansion mark the constant as Opaque and stop DAGCombiner from constant folding in this situation. (Similar to how ConstantHoisting marks things as Opaque to avoid folding ADD/SUB/etc.) rdar://44153998

When implementing memset's today we often see this pattern: $x0 = MOV 0xXYXYXYXYXYXYXYXY store $x0, ... $w1 = MOV 0xXYXYXYXY store $w1, ... We first create a 64bit constant in a 64bit register with all bytes the same and then create a 32bit constant with all bytes the same in a 32bit register. In many targets we could just access the lower part of the 64bit register instead. - Ideally this would be handled by the ConstantHoist pass but it runs too early when memset isn't expanded yet. - The memset expansion code already had this optimization implemented, however it didn't work as DAGCombiner would fold "trunc(bigconstant)" back to "smallconstant". - This patch makes the memset expansion mark the constant as Opaque and stop DAGCombiner from constant folding in this situation. (Similar to how ConstantHoisting marks things as Opaque to avoid folding ADD/SUB/etc.) rdar://44153998

When implementing memset's today we often see this pattern: $x0 = MOV 0xXYXYXYXYXYXYXYXY store $x0, ... $w1 = MOV 0xXYXYXYXY store $w1, ... We first create a 64bit constant in a 64bit register with all bytes the same and then create a 32bit constant with all bytes the same in a 32bit register. In many targets we could just access the lower bytepart of the 64bit register instead. - Ideally this would be handled by the ConstantHoist pass but it runs too early when memset isn't expanded yet. - The memset expansion code already had this optimization implemented, however it didn't work as DAGCombiner would fold "trunc(bigconstant)" back to "smallconstant". - This patch makes the memset expansion mark the constant as Opaque and stop DAGCombiner from constant folding in this situation. (Similar to how ConstantHoisting marks things as Opaque to avoid folding ADD/SUB/etc.) rdar://44153998