From a4f468f245d6e6869317007c548ee4d33ad97343 Mon Sep 17 00:00:00 2001 From: Tom Stellard Date: Wed, 16 Oct 2013 17:06:02 +0000 Subject: R600: Fix a crash in the AMDILCFGStructurizer We were calling llvm_unreachable() when failing to optimize the branch into if case. However, it is still possible for us to structurize the CFG by duplicating blocks even if this optimization fails. Reviewed-by: Vincent Lejeune git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192813 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Target/R600/AMDILCFGStructurizer.cpp | 70 +++++++++++++++++++++++++++++++- 1 file changed, 68 insertions(+), 2 deletions(-) (limited to 'lib/Target/R600/AMDILCFGStructurizer.cpp') diff --git a/lib/Target/R600/AMDILCFGStructurizer.cpp b/lib/Target/R600/AMDILCFGStructurizer.cpp index f88d593ef7..a9337f0562 100644 --- a/lib/Target/R600/AMDILCFGStructurizer.cpp +++ b/lib/Target/R600/AMDILCFGStructurizer.cpp @@ -1335,8 +1335,74 @@ int AMDGPUCFGStructurizer::improveSimpleJumpintoIf(MachineBasicBlock *HeadMBB, // add initReg = initVal to headBlk const TargetRegisterClass * I32RC = TRI->getCFGStructurizerRegClass(MVT::i32); - if (!MigrateTrue || !MigrateFalse) - llvm_unreachable("Extra register needed to handle CFG"); + if (!MigrateTrue || !MigrateFalse) { + // XXX: We have an opportunity here to optimize the "branch into if" case + // here. Branch into if looks like this: + // entry + // / \ + // diamond_head branch_from + // / \ | + // diamond_false diamond_true + // \ / + // done + // + // The diamond_head block begins the "if" and the diamond_true block + // is the block being "branched into". + // + // If MigrateTrue is true, then TrueBB is the block being "branched into" + // and if MigrateFalse is true, then FalseBB is the block being + // "branched into" + // + // Here is the pseudo code for how I think the optimization should work: + // 1. Insert MOV GPR0, 0 before the branch instruction in diamond_head. + // 2. Insert MOV GPR0, 1 before the branch instruction in branch_from. + // 3. Move the branch instruction from diamond_head into its own basic + // block (new_block). + // 4. Add an unconditional branch from diamond_head to new_block + // 5. Replace the branch instruction in branch_from with an unconditional + // branch to new_block. If branch_from has multiple predecessors, then + // we need to replace the True/False block in the branch + // instruction instead of replacing it. + // 6. Change the condition of the branch instruction in new_block from + // COND to (COND || GPR0) + // + // In order insert these MOV instruction, we will need to use the + // RegisterScavenger. Usually liveness stops being tracked during + // the late machine optimization passes, however if we implement + // bool TargetRegisterInfo::requiresRegisterScavenging( + // const MachineFunction &MF) + // and have it return true, liveness will be tracked correctly + // by generic optimization passes. We will also need to make sure that + // all of our target-specific passes that run after regalloc and before + // the CFGStructurizer track liveness and we will need to modify this pass + // to correctly track liveness. + // + // After the above changes, the new CFG should look like this: + // entry + // / \ + // diamond_head branch_from + // \ / + // new_block + // / \ + // diamond_false diamond_true + // \ / + // done + // + // Without this optimization, we are forced to duplicate the diamond_true + // block and we will end up with a CFG like this: + // + // entry + // / \ + // diamond_head branch_from + // / \ | + // diamond_false diamond_true diamond_true (duplicate) + // \ / | + // done --------------------| + // + // Duplicating diamond_true can be very costly especially if it has a + // lot of instructions. + return 0; + } int NumNewBlk = 0; -- cgit v1.2.3