[NFCI][TableGen][DecoderEmitter] Cull Op handling when possible #142974
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jurahul
changed the title
TryDecode when possibleTryDecode handling when possible
Member
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@llvm/pr-subscribers-tablegen Author: Rahul Joshi (jurahul) Changes
Full diff: https://github.com/llvm/llvm-project/pull/142974.diff 1 Files Affected:
diff --git a/llvm/utils/TableGen/DecoderEmitter.cpp b/llvm/utils/TableGen/DecoderEmitter.cpp
index 3990836e9077f..ec12eab8ac4ca 100644
--- a/llvm/utils/TableGen/DecoderEmitter.cpp
+++ b/llvm/utils/TableGen/DecoderEmitter.cpp
@@ -223,8 +223,9 @@ class DecoderEmitter {
DecoderEmitter(const RecordKeeper &R, StringRef PredicateNamespace)
: RK(R), Target(R), PredicateNamespace(PredicateNamespace) {}
- // Emit the decoder state machine table.
- void emitTable(formatted_raw_ostream &OS, DecoderTable &Table, indent Indent,
+ // Emit the decoder state machine table. Return true if any `TryDecode` ops
+ // were generated.
+ bool emitTable(formatted_raw_ostream &OS, DecoderTable &Table, indent Indent,
unsigned BitWidth, StringRef Namespace,
const EncodingIDsVec &EncodingIDs) const;
void emitInstrLenTable(formatted_raw_ostream &OS,
@@ -828,7 +829,7 @@ unsigned Filter::usefulness() const {
//////////////////////////////////
// Emit the decoder state machine table.
-void DecoderEmitter::emitTable(formatted_raw_ostream &OS, DecoderTable &Table,
+bool DecoderEmitter::emitTable(formatted_raw_ostream &OS, DecoderTable &Table,
indent Indent, unsigned BitWidth,
StringRef Namespace,
const EncodingIDsVec &EncodingIDs) const {
@@ -885,6 +886,8 @@ void DecoderEmitter::emitTable(formatted_raw_ostream &OS, DecoderTable &Table,
OS << " (Fail)";
};
+ bool HasTryDecode = false;
+
while (I != E) {
assert(I < E && "incomplete decode table entry!");
@@ -965,6 +968,7 @@ void DecoderEmitter::emitTable(formatted_raw_ostream &OS, DecoderTable &Table,
case MCD::OPC_TryDecodeOrFail: {
bool IsFail = DecoderOp == MCD::OPC_TryDecodeOrFail;
bool IsTry = DecoderOp == MCD::OPC_TryDecode || IsFail;
+ HasTryDecode |= IsTry;
// Decode the Opcode value.
const char *ErrMsg = nullptr;
unsigned Opc = decodeULEB128(&*I, nullptr, EndPtr, &ErrMsg);
@@ -1027,6 +1031,8 @@ void DecoderEmitter::emitTable(formatted_raw_ostream &OS, DecoderTable &Table,
Indent -= 2;
OS << Indent << "};\n\n";
+
+ return HasTryDecode;
}
void DecoderEmitter::emitInstrLenTable(formatted_raw_ostream &OS,
@@ -2217,8 +2223,8 @@ static void insertBits(InsnType &field, InsnType bits, unsigned startBit,
// emitDecodeInstruction - Emit the templated helper function
// decodeInstruction().
-static void emitDecodeInstruction(formatted_raw_ostream &OS,
- bool IsVarLenInst) {
+static void emitDecodeInstruction(formatted_raw_ostream &OS, bool IsVarLenInst,
+ bool HasTryDecode) {
OS << R"(
static unsigned decodeNumToSkip(const uint8_t *&Ptr) {
unsigned NumToSkip = *Ptr++;
@@ -2363,7 +2369,9 @@ static DecodeStatus decodeInstruction(const uint8_t DecodeTable[], MCInst &MI,
<< ", using decoder " << DecodeIdx << ": "
<< (S != MCDisassembler::Fail ? "PASS\n" : "FAIL\n"));
return S;
- }
+ })";
+ if (HasTryDecode) {
+ OS << R"(
case MCD::OPC_TryDecode:
case MCD::OPC_TryDecodeOrFail: {
bool IsFail = DecoderOp == MCD::OPC_TryDecodeOrFail;
@@ -2398,7 +2406,9 @@ static DecodeStatus decodeInstruction(const uint8_t DecodeTable[], MCInst &MI,
// set before the decode attempt.
S = MCDisassembler::Success;
break;
- }
+ })";
+ }
+ OS << R"(
case MCD::OPC_SoftFail: {
// Decode the mask values.
uint64_t PositiveMask = decodeULEB128AndIncUnsafe(Ptr);
@@ -2608,6 +2618,7 @@ namespace {
}
DecoderTableInfo TableInfo;
+ bool HasTryDecode = false;
for (const auto &Opc : OpcMap) {
// Emit the decoder for this namespace+width combination.
ArrayRef<EncodingAndInst> NumberedEncodingsRef(NumberedEncodings.data(),
@@ -2633,8 +2644,8 @@ namespace {
TableInfo.Table.push_back(MCD::OPC_Fail);
// Print the table to the output stream.
- emitTable(OS, TableInfo.Table, indent(0), FC.getBitWidth(), Opc.first.first,
- Opc.second);
+ HasTryDecode |= emitTable(OS, TableInfo.Table, indent(0), FC.getBitWidth(),
+ Opc.first.first, Opc.second);
}
// For variable instruction, we emit a instruction length table
@@ -2649,7 +2660,7 @@ namespace {
emitDecoderFunction(OS, TableInfo.Decoders, indent(0));
// Emit the main entry point for the decoder, decodeInstruction().
- emitDecodeInstruction(OS, IsVarLenInst);
+ emitDecodeInstruction(OS, IsVarLenInst, HasTryDecode);
OS << "\n} // namespace\n";
}
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Collaborator
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Is there a measurable benefit to this? |
Contributor
Author
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I did not do any measurements as it seemed simple to implement. The only benefit will be a minor reduction in code size as this eliminates dead code from each target's disassembler code. I suppose |
`TryDecode` MCD ops are not used by many targets (only AArch64 seems to generate it). Track whether any `TryDecode` or `TryDecodeOrFail` ops are generated and emit the code for handling them only in that case.
Contributor
Author
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Here are the code size reductions based on a modified version of this: |
Contributor
Author
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Thanks. I have a new version that culls more with which I got the above results. Will ask for another review once that passes checks. |
jurahul
changed the title
TryDecode handling when possible
Contributor
Author
|
I've now uploaded the new changes that does more aggressive culling (as a separate commit). @topperc this is what I used to get the results above. Can you PTAL again? |
topperc
reviewed
Jun 17, 2025
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TryDecode/CheckPredicate/SoftFail MCD ops are not used by many targets. Track the set of opcodes that were emitted and emit code for handling TryDecode/CheckPredicate/SoftFail ops when decoding only if there were emitted. This is purely eliminating dead code in the generated
decodeInstructionfunction.This results in the following reduction in the size of the Disassembler .so files with a release x86_64 release build on Linux: