This is the parent patch of a series of patches to enable Basic Block Sections support in LLVM which is the building block for the Propeller post link optimization framework. Please see the RFC here: https://groups.google.com/forum/#!msg/llvm-dev/ef3mKzAdJ7U/1shV64BYBAAJ and the detailed RFC doc here: https://github.com/google/llvm-propeller/blob/plo-dev/Propeller_RFC.pdf.
We introduce a new compiler option, -fbasicblock-sections=, which places every basic block in a unique ELF text section in the object file along with a symbol labeling the basic block. The linker can then order the basic block sections in any arbitrary sequence which when done correctly can encapsulate block layout, function layout and function splitting optimizations. However, there are a couple of challenges to be addressed for this to be feasible:
- The compiler must not allow any implicit fall-through between any two adjacent basic blocks as they could be reordered at link time to be non-adjacent. In other words, the compiler must make a fall-through between adjacent basic blocks explicit by retaining the direct jump instruction that jumps to the next basic block. These branches can only be removed later by the linker after the blocks have been reordered.
- All inter-basic block branch targets would now need to be resolved by the linker as they cannot be calculated during compile time. This is done using static relocations which bloats the size of the object files. Further, the compiler tries to use short branch instructions on some ISAs for branch offsets that can be accommodated in one byte. This is not possible with basic block sections as the offset is not determined at compile time, and long branch instructions have to be used everywhere.
- Each additional section bloats object file sizes by tens of bytes. The number of basic blocks can be potentially very large compared to the size of functions and can bloat object sizes significantly. Option fbasicblock-sections= also takes a file path which can be used to specify a subset of basic blocks that needs unique sections to keep the bloats small.
- Debug Info and CFI need special handling and will be presented as separate patches.
Basic Block Labels
With -fbasicblock-sections=labels, or when a basic block is placed in a unique section, it is labelled with a symbol. This allows easy mapping of virtual addresses from
PMU profiles back to the corresponding basic blocks. Since the number of basic blocks is large, the labeling bloats the symbol table sizes and the string table sizes significantly. While the binary size does increase, it does not affect performance as the symbol table is not loaded in memory during run-time. The string table size bloat is kept very minimal using a unary naming scheme that uses string suffix compression. The basic blocks for function foo are named "a.BB.foo", "aa.BB.foo", ... This turns out to be very good for string table sizes and the bloat in the string table size for a very large binary is ~8 %. The naming also allows using the --symbol-ordering-file option in LLD to arbitrarily reorder the sections.