Add HexagonGlobalScheduler pass (#180803)

This patch adds the HexagonGlobalScheduler, a post-packetization pass
that performs global instruction scheduling and pull-up optimizations to
improve packet density on Hexagon VLIW architecture.

The scheduler operates on scheduling regions (super-blocks with single
entry and multiple exits) and attempts to move instructions across basic
blocks to fill packet slots more efficiently. It supports both
speculative and predicative scheduling modes.

Key features:
- Global instruction scheduling across basic blocks
- Speculative scheduling with safety checks
- Predicative scheduling using predication
- Local pull-up within basic blocks
- Dual jump formation
- Branch optimizations
- Liveness preservation using HexagonLiveVariables

The pass depends on HexagonLiveVariables and HexagonGlobalRegion
infrastructure for maintaining correct liveness information when moving
instructions across basic block boundaries.

This patch also adds two helper functions to HexagonRegisterInfo that
are required by the scheduler to prevent correctness issues.

1. isGlobalReg(MCPhysReg Reg) - Returns true for reserved physical
registers (R29, R30, R31) that are live across function calls. This
prevents the scheduler from incorrectly speculating instructions that
modify the stack pointer or frame pointer.

2. isFakeReg(MCPhysReg Reg) - Returns true for fake HVX registers
(VF0-VF31, VFR0-VFR31) used as sub-registers in vector pairs. This
prevents crashes when the scheduler processes HVX vector operations by
skipping these internal bookkeeping registers.

The scheduler runs in the pre-emit pipeline after packetization and is
only active when optimizations are enabled.

Patch by Sergei Larin <slarin@qti.qualcomm.com>

Co-authored-by: Ikhlas Ajbar <iajbar@qti.qualcomm.com>
Co-authored-by: Krzysztof Parzyszek <kparzysz@codeaurora.org>

Author: 3 people

llvm/lib/Target/Hexagon/CMakeLists.txt

@@ -39,6 +39,7 @@ add_llvm_target(HexagonCodeGen
   HexagonGenMux.cpp
   HexagonGenPredicate.cpp
   HexagonGlobalRegion.cpp
+  HexagonGlobalScheduler.cpp
   HexagonLiveVariables.cpp
   HexagonHardwareLoops.cpp
   HexagonHazardRecognizer.cpp

llvm/lib/Target/Hexagon/Hexagon.h

@@ -41,6 +41,7 @@ void initializeHexagonEarlyIfConversionPass(PassRegistry &);
 void initializeHexagonExpandCondsetsPass(PassRegistry &);
 void initializeHexagonGenMemAbsolutePass(PassRegistry &);
 void initializeHexagonGenMuxPass(PassRegistry &);
+void initializeHexagonGlobalSchedulerPass(PassRegistry &);
 void initializeHexagonHardwareLoopsPass(PassRegistry &);
 void initializeHexagonLiveVariablesPass(PassRegistry &);
 void initializeHexagonLoopIdiomRecognizeLegacyPassPass(PassRegistry &);
@@ -94,6 +95,7 @@ FunctionPass *createHexagonGenInsert();
 FunctionPass *createHexagonGenMemAbsolute();
 FunctionPass *createHexagonGenMux();
 FunctionPass *createHexagonGenPredicate();
+FunctionPass *createHexagonGlobalScheduler();
 FunctionPass *createHexagonHardwareLoops();
 FunctionPass *createHexagonISelDag(HexagonTargetMachine &TM,
                                    CodeGenOptLevel OptLevel);