This change adds an option to specialize decoders per bitwidth, which
can help reduce the (compiled) code size of the decoder code.
**Current state**:
Currently, the code generated by the decoder emitter consists of two key
functions: `decodeInstruction` which is the entry point into the
generated code and `decodeToMCInst` which is invoked when a decode op is
reached while traversing through the decoder table. Both functions are
templated on `InsnType` which is the raw instruction bits that are
supplied to `decodeInstruction`.
Several backends call `decodeInstruction` with different `InsnType`
types, leading to several template instantiations of these functions in
the final code. As an example, AMDGPU instantiates this function with
type `DecoderUInt128` type for decoding 96/128-bit instructions,
`uint64_t` for decoding 64-bit instructions, and `uint32_t` for decoding
32-bit instructions. Since there is just one `decodeToMCInst` in the
generated code, it has code that handles decoding for *all* instruction
sizes. However, the decoders emitted for different instructions sizes
rarely have any intersection with each other. That means, in the AMDGPU
case, the instantiation with InsnType == DecoderUInt128 has decoder code
for 32/64-bit instructions that is *never exercised*. Conversely, the
instantiation with InsnType == uint64_t has decoder code for
128/96/32-bit instructions that is never exercised. This leads to
unnecessary dead code in the generated disassembler binary (that the
compiler cannot eliminate by itself).
**New state**:
With this change, we introduce an option
`specialize-decoders-per-bitwidth`. Under this mode, the DecoderEmitter
will generate several versions of `decodeToMCInst` function, one for
each bitwidth. The code is still templated, but will require backends to
specify, for each `InsnType` used, the bitwidth of the instruction that
the type is used to represent using a type-trait `InsnBitWidth`. This
will enable the templated code to choose the right variant of
`decodeToMCInst`. Under this mode, a particular instantiation will only
end up instantiating a single variant of `decodeToMCInst` generated and
that will include only those decoders that are applicable to a single
bitwidth, resulting in elimination of the code duplication through
instantiation and a reduction in code size.
Additionally, under this mode, decoders are uniqued only within a given
bitwidth (as opposed to across all bitwidths without this option), so
the decoder index values assigned are smaller, and consume less bytes in
their ULEB128 encoding. As a result, the generated decoder tables can
also reduce in size.
Adopt this feature for the AMDGPU and RISCV backend. In a release build,
this results in a net 55% reduction in the .text size of
libLLVMAMDGPUDisassembler.so and a 5% reduction in the .rodata size. For
RISCV, which today uses a single `uint64_t` type, this results in a 3.7%
increase in code size (expected as we instantiate the code 3 times now).
Actual measured sizes are as follows:
```
Baseline commit: 72c04bb882ad70230bce309c3013d9cc2c99e9a7
Configuration: Ubuntu clang version 18.1.3, release build with asserts disabled.
AMDGPU Before After Change
======================================================
.text 612327 275607 55% reduction
.rodata 369728 351336 5% reduction
RISCV:
======================================================
.text 47407 49187 3.7% increase
.rodata 35768 35839 0.1% increase
```
Add option `use-fn-table-in-decode-to-mcinst` to use a table of function
pointers instead of a switch case in the generated `decodeToMCInst`
function.
When the number of switch cases in this function is large, the generated
code takes a long time to compile in release builds. Using a table of
function pointers instead improves the compile time significantly (~3x
speedup in compiling the code in a downstream target). This option will
allow targets to opt into this mode if they desire for better build
times.
Tested with `check-llvm-mc` with the option enabled by default.
