[llvm] [LLVM][MC] Add support to cull inactive decoders in decoder emitter (PR #154865)
Sergei Barannikov via llvm-commits
llvm-commits at lists.llvm.org
Mon Aug 25 23:25:19 PDT 2025
================
@@ -2528,45 +2535,80 @@ namespace {
)";
// Do extra bookkeeping for variable-length encodings.
- std::vector<unsigned> InstrLen;
bool IsVarLenInst = Target.hasVariableLengthEncodings();
unsigned MaxInstLen = 0;
if (IsVarLenInst) {
- InstrLen.resize(Target.getInstructions().size(), 0);
+ std::vector<unsigned> InstrLen(Target.getInstructions().size(), 0);
for (const InstructionEncoding &Encoding : Encodings) {
MaxInstLen = std::max(MaxInstLen, Encoding.getBitWidth());
InstrLen[Target.getInstrIntValue(Encoding.getInstruction()->TheDef)] =
Encoding.getBitWidth();
}
+
+ // For variable instruction, we emit a instruction length table to let the
+ // decoder know how long the instructions are. You can see example usage in
+ // M68k's disassembler.
+ emitInstrLenTable(OS, InstrLen);
}
- // Map of (namespace, hwmode, size) tuple to encoding IDs.
- std::map<std::tuple<StringRef, unsigned, unsigned>, std::vector<unsigned>>
+ // Map of (bitwidth, namespace, hwmode) tuple to encoding IDs.
+ std::map<std::tuple<unsigned, StringRef, unsigned>, std::vector<unsigned>>
----------------
s-barannikov wrote:
I tried a map of maps and this is how it looks like (didn't check that it generates correct code):
<details>
<summary>looks much cleaner to me</summary>
```C++
for (const auto &[BitWidth, BWMap] : EncMap) {
for (const auto &[Key, EncodingIDs] : BWMap) {
auto [DecoderNamespace, HwModeID] = Key;
// Emit the decoder for this (namespace, hwmode, width) combination.
FilterChooser FC(Encodings, EncodingIDs, BitWidth, Target);
// The decode table is cleared for each top level decoder function. The
// predicates and decoders themselves, however, are shared across
// different decoders to give more opportunities for uniqueing.
// - If `SpecializeDecodersPerBitwidth` is enabled, decoders are shared
// across all decoder tables for a given bitwidth, else they are shared
// across all decoder tables.
// - predicates are shared across all decoder tables.
TableInfo.Table.clear();
TableBuilder.buildTable(FC);
// Print the table to the output stream.
OpcodeMask |= emitTable(OS, TableInfo.Table, DecoderNamespace, HwModeID,
BitWidth, EncodingIDs);
}
if (SpecializeDecodersPerBitwidth) {
emitDecoderFunction(OS, TableInfo.Decoders, BitWidth);
// Each BitWidth get's its own decoders and decoder function.
TableInfo.Decoders.clear();
}
}
// Emit the decoder function for the last bucket. This will also emit the
// single decoder function if SpecializeDecodersPerBitwidth = false.
if (!SpecializeDecodersPerBitwidth)
emitDecoderFunction(OS, TableInfo.Decoders, 0);
```
</details>
WDYT?
https://github.com/llvm/llvm-project/pull/154865
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