Taichi

- [doc] Minor updates in `hello.rst` (963) (by **Yuanming Hu**)
- [doc] Improve `hello.rst` (972) (by **Xiao Zhai**)
- [misc] Add a link to taichi_elements in README.md (974) (by **彭于斌**)
- [Bug] [opengl] [llvm] [metal] Fix floordiv overflow when `lhs * rhs` is large (970) (by **彭于斌**)
- [test] Size Performance Regression Tests (SPRT) (959) (by **彭于斌**)
- [Metal] Skip allocating Runtime::ListManager if no sparse SNode is used (976) (by **Ye Kuang**)
- [cuda] Improve kernel return value performance when unified memory is available (965) (by **Yuanming Hu**)
- [doc] Update `syntax.rst` and related sections (967) (by **Yuanming Hu**)
- [opengl] add TI_WITH_OPENGL env var to disable OpenGL (962) (by **彭于斌**)
- [opt] [cuda] Fix constant folding in multithreaded CUDA applications (954) (by **Yuanming Hu**)
- [ir][refactor] Avoid throwing exception in `replace_statements_with` (968) (by **xumingkuan**)
- [opengl] Use `compile_to_offloads` for IR lowering (951) (by **彭于斌**)
- [Doc] fix typo in syntax_sugars.po (960) (by **Danni**)
- [refactor][ir] Simplify ir.h to reduce compile-time (934) (by **彭于斌**)
- [lang] [refactor] deprecate boardcast_if_scalar, all use binary and unary (943) (by **彭于斌**)
- [doc] Improve `hello.rst` (955) (by **Yuanming Hu**)
- [metal] Set labels on the command buffer/encoder (950) (by **Ye Kuang**)
- [ir] Remove BasicStmtVisitor::current_struct_for (952) (by **xumingkuan**)
- [misc] README.md use shields.io to show latest release (947) (by **彭于斌**)
- [Lang][IR][refactor] Refactor SNode and support coordinate offsets (945) (by **Yuanming Hu**)

- [Doc] Add documentions for layout (900) (by **彭于斌**)
- [IR] Use JIT compilation/evaluation for systematic constant folding (839) (by **彭于斌**)
- [doc] Update doc for kernel return in 930 (942) (by **彭于斌**)
- [Lang] `ti.sqr(x)` is now deprecated: please use `x ** 2` instead (937) (by **Yuanming Hu**)
- [ir][refactor] Deprecate `ArgStoreStmt` (both frontend and CHI) (936) (by **Ye Kuang**)
- [metal] Support `KernelReturnStmt` (930) (by **Ye Kuang**)
- [Misc] Remove dependency on glibc 2.27 and improve portability (931) (by **Yuanming Hu**)
- [Lang] [IR] Kernel scalar `return` support (`ArgStoreStmt` -> `KernelReturnStmt`) (917) (by **彭于斌**)
- [Example] Add `waterwave.py` (792) (by **彭于斌**)

Highlights:
- **Miscellaneous**
- Add `ti.core.toggle_advanced_optimization(True/False)` (927) (by **Yuanming Hu**)

Full changelog:
- [doc] Non-Ubuntu Linux users are suggested to build clang-8 from scratch (867) (by **彭于斌**)
- [Misc] Add `ti.core.toggle_advanced_optimization(True/False)` (927) (by **Yuanming Hu**)
- [ir][refactor] Move legacy frontend constructs to frontend.h/cpp (924) (by **Ye Kuang**)
- [misc] Enforce format (922) (by **Taichi Gardener**)
- [ir][refactor] Move all Expression subclasses to `frontend_ir.h` (919) (by **Ye Kuang**)

Highlights:
- **Automatic differentiation**
- Fix CUDA data layout and stack alignment (918) (by **Yuanming Hu**)
- **CUDA backend**
- Fix CUDA data layout and stack alignment (918) (by **Yuanming Hu**)
- **Examples**
- Add `example/bitmasked.py` (905) (by **彭于斌**)
- **GUI**
- Better event filtering system (801) (by **彭于斌**)
- **Language and syntax**
- Scalar math functions can now be applied element-wisely to vectors/matrices (891) (by **彭于斌**)

Full changelog:
- [ir][refactor] Move all frontend stmts to `frontend_ir.h` (916) (by **Ye Kuang**)
- [CUDA] [AutoDiff] Fix CUDA data layout and stack alignment (918) (by **Yuanming Hu**)
- [Example] Add example/bitmasked.py (905) (by **彭于斌**)
- [opengl] [bug] Move GLSLRuntime into a separate buffer, fix ti.random() bug on NVIDIA (912) (by **彭于斌**)
- [ir][refactor] First step to move Frontend IR to its own file (914) (by **Ye Kuang**)
- [async] Avoid unnecessary list generations and activations (913) (by **Yuanming Hu**)
- [Lang] Scalar math functions can now be applied element-wisely to vectors/matrices (891) (by **彭于斌**)
- [ir] Deprecate `FrontendAtomicStmt` (907) (by **Ye Kuang**)
- [ir] [refactor] Remove the global `current_block` (908) (by **Ye Kuang**)
- [ir][refactor] Pass a context object to `Expression::flatten()` (901) (by **Ye Kuang**)
- [test] Add an argument -r to rerun failed test (904) (by **彭于斌**)
- [GUI] Better event filtering system (801) (by **彭于斌**)

Highlights:
- **The OpenGL Compute Shader backend (by **彭于斌**) is officially released!**
- **Examples**
- Add Multiple Importance Sampling to the cornell box scene (890) (by **Ye Kuang**)
- **IR optimization passes**
- Extract constants to top-level (897) (by **xumingkuan**)
- Global variable optimizations (857) (by **xumingkuan**)

Full changelog:
- [ir] [refactor] Separate `UnaryOpType::cast` into `cast_value` and `cast_bits` (892) (by **彭于斌**)
- [opt] Set `has_global_side_effect` to `false` for some statements (898) (by **xumingkuan**)
- [Example] Add Multiple Importance Sampling to the cornell box scene (890) (by **Ye Kuang**)
- [async] Overlap compilation and execution (885) (by **Yuanming Hu**)
- [Opt] Extract constants to top-level (897) (by **xumingkuan**)
- [ir] Remove `Ident` alias (895) (by **Ye Kuang**)
- [opt] Use advanced_optimization for 857 (894) (by **xumingkuan**)
- [Opt] Global variable optimizations (857) (by **xumingkuan**)

- [Bug] Fix CPU SNode reader in debug mode (887) (by **Yuanming Hu**)
- [opengl] Tolerate all errors in with_opengl() (884) (by **彭于斌**)
- [Opt] [Refactor] [Bug] Use state machines for optimization on variables (859) (by **xumingkuan**)
- [async] No sync after kernel launch and postpone lower access (883) (by **Yuanming Hu**)
- [async] Fix SNode reading in async mode (881) (by **Yuanming Hu**)
- [opt] Eliminate assertions with non-zero const conditions (877) (by **xumingkuan**)
- [misc] Reorganize misc files (876) (by **Yuanming Hu**)
- revert 769 ti.cfg.gdb_trigger error (879) (by **彭于斌**)
- [cuda] [test] Add TI_DEVICE_MEMORY_GB and TI_DEVICE_MEMORY_FRACTION environment variable (769) (by **彭于斌**)
- [OpenGL] Detect driver existence in `with_opengl()` (864) (by **彭于斌**)
- [misc] Avoid unnecessary linking when git commit hash not changed (871) (by **Yuanming Hu**)
- [async] [refactor] Fix async engine statement not found error (874) (by **Yuanming Hu**)
- [example] Fix `pbf2d.py`'s initialization to use np.float32 (873) (by **Ye Kuang**)
- [misc] Avoid mutable defaults in `PyTaichi` (870) (by **Yuxin Wu**)
- [misc] Fix imread not reading properly when image is not equal in height and width (855) (by **彭于斌**)
- [async] Parallel compilation (863) (by **Yuanming Hu**)
- [Opt] [Bug] Fix the state of the last store after a loop (862) (by **xumingkuan**)
- [test] `ti test -na opengl` to exclude tests on some archs (830) (by **彭于斌**)
- [opt] Eliminate useless local stores and atomics (858) (by **xumingkuan**)
- [doc] update documention for 843 (844) (by **彭于斌**)
- [Example] add sdf2d.py (835) (by **彭于斌**)
- [opt] Lower `linearize` even without advanced optimization (854) (by **xumingkuan**)
- [misc] Split accessor and other kernels in statistics (853) (by **xumingkuan**)
- [misc] Profiler now supports multiple threads (852) (by **Yuanming Hu**)
- [opt] Improve optimization for OffsetAndExtractBitsStmt (851) (by **xumingkuan**)
- [Bug] [Opt] Fix a bug in local variable optimization (849) (by **xumingkuan**)
- [test] Fix duplicate test name in `test_tensor_reflection.py` (850) (by **xumingkuan**)
- [Example] Add `quadtree.py` (824) (by **彭于斌**)
- [misc] Avoid printing empty logs (848) (by **xumingkuan**)
- [async] Each JIT thread now has its own LLVM context (845) (by **Yuanming Hu**)
- [misc] Clear stats in the constructor of Program (by **xumingkuan**)
- [Misc] Enhance `ti.init` and support `arch=ti.gpu` (843) (by **彭于斌**)
- [misc] Add an option to print statistics into files (841) (by **xumingkuan**)
- [test] Fix test when `advanced_optimization == false` (842) (by **xumingkuan**)
- [Example] Add a Cornell box renderer (836) (by **Ye Kuang**)
- [metal] Increment by thread grid size for the grid-strip loops (837) (by **Ye Kuang**)
- [OpenGL] Use GLAD as API loader (replace GLEW) (819) (by **彭于斌**)
- [opt] Eliminate `WhileControlStmt` with non-zero const conditions (by **xumingkuan**)
- [refactor] Move backend implementations to the `backends` folder (818) (by **Yuanming Hu**)
- [test] Add a test for statement offloading (826) (by **xumingkuan**)
- [Opt] Algebraic simplification for bitwise operators (827) (by **xumingkuan**)