From 72515bef32d5e17a3fccc9cf4f06bc39d91ee4c6 Mon Sep 17 00:00:00 2001 From: Andrew Trick Date: Wed, 14 Mar 2012 04:00:38 +0000 Subject: [PATCH] misched comments llvm-svn: 152699 --- llvm/lib/CodeGen/MachineScheduler.cpp | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/llvm/lib/CodeGen/MachineScheduler.cpp b/llvm/lib/CodeGen/MachineScheduler.cpp index 8b70ae5350d4..bb4b89fd7fa9 100644 --- a/llvm/lib/CodeGen/MachineScheduler.cpp +++ b/llvm/lib/CodeGen/MachineScheduler.cpp @@ -113,6 +113,21 @@ SchedDefaultRegistry("default", "Use the target's default scheduler choice.", /// default scheduler if the target does not set a default. static ScheduleDAGInstrs *createCommonMachineSched(MachineSchedContext *C); +/// Top-level MachineScheduler pass driver. +/// +/// Visit blocks in function order. Divide each block into scheduling regions +/// and visit them bottom-up. This is consistent with the DAG builder, which +/// traverses scheduling regions bottom-up, but not essential. +/// +/// This design avoids exposing scheduling boundaries to the DAG builder, +/// simplifying the DAG builder's support for "special" target instructions, +/// while at the same time allowing target schedulers to operate across +/// scheduling boundaries, for example to bundle the boudary instructions +/// without reordering them. This creates complexity, because the target +/// scheduler must update the RegionBegin and RegionEnd positions cached by +/// ScheduleDAGInstrs whenever adding or removing instructions. A much simpler +/// design would be to split blocks at scheduling boundaries, but LLVM has a +/// general bias against block splitting purely for implementation simplicity. bool MachineScheduler::runOnMachineFunction(MachineFunction &mf) { // Initialize the context of the pass. MF = &mf;