60bdf09654
### Current state We have FilterChooser class, which can be thought of as a **tree of encodings**. Tree nodes are instances of FilterChooser itself, and come in two types: * A node containing single encoding that has *constant* bits in the specified bit range, a.k.a. singleton node. * A node containing only child nodes, where each child represents a set of encodings that have the same *constant* bits in the specified bit range. Either of these nodes can have an additional child, which represents a set of encodings that have some *unknown* bits in the same bit range. As can be seen, the **data structure is very high level**. The encoding tree represented by FilterChooser is then converted into a finite-state machine (FSM), represented as **byte array**. The translation is straightforward: for each node of the tree we emit a sequence of opcodes that check encoding bits and predicates for each encoding. For a singleton node we also emit a terminal "decode" opcode. The translation is done in one go, and this has negative consequences: * We miss optimization opportunities. * We have to use "fixups" when encoding transitions in the FSM since we don't know the size of the data we want to jump over in advance. We have to emit the data first and then fix up the location of the jump. This means the fixup size has to be large enough to encode the longest jump, so **most of the transitions are encoded inefficiently**. * Finally, when converting the FSM into human readable form, we have to **decode the byte array we've just emitted**. This is also done in one go, so we **can't do any pretty printing**. ### This PR We introduce an intermediary data structure, decoder tree, that can be thought as **AST of the decoder program**. This data structure is **low level** and as such allows for optimization and analysis. It resolves all the issues listed above. We now can: * Emit more optimal opcode sequences. * Compute the size of the data to be emitted in advance, avoiding fixups. * Do pretty printing. Serialization is done by a new class, DecoderTableEmitter, which converts the AST into a FSM in **textual form**, streamed right into the output file. ### Results * The new approach immediately resulted in 12% total table size savings across all in-tree targets, without implementing any optimizations on the AST. Many tables observe ~20% size reduction. * The generated file is much more readable. * The implementation is arguably simpler and more straightforward (the diff is only +150~200 lines, which feels rather small for the benefits the change gives).
119 lines
3.4 KiB
TableGen
119 lines
3.4 KiB
TableGen
// RUN: llvm-tblgen -gen-disassembler -I %p/../../include %s | \
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// RUN: FileCheck %s --check-prefix=DECODER
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// RUN: llvm-tblgen -gen-disassembler --suppress-per-hwmode-duplicates=O2 -I \
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// RUN: %p/../../include %s | FileCheck %s --check-prefix=DECODER-SUPPRESS
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// Test duplicate table suppression for per-HwMode decoders.
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include "llvm/Target/Target.td"
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def archInstrInfo : InstrInfo { }
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def arch : Target {
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let InstructionSet = archInstrInfo;
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}
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def Myi32 : Operand<i32> {
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let DecoderMethod = "DecodeMyi32";
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}
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def HasA : Predicate<"Subtarget->hasA()">;
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def HasB : Predicate<"Subtarget->hasB()">;
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def ModeA : HwMode<[HasA]>;
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def ModeB : HwMode<[HasB]>;
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def fooTypeEncA : InstructionEncoding {
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let Size = 4;
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field bits<32> SoftFail = 0;
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bits<32> Inst;
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bits<8> factor;
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let Inst{7...0} = factor;
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let Inst{3...2} = 0b11;
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let Inst{1...0} = 0b00;
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}
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def fooTypeEncB : InstructionEncoding {
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let Size = 4;
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field bits<32> SoftFail = 0;
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bits<32> Inst;
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bits<8> factor;
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let Inst{15...8} = factor;
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let Inst{1...0} = 0b11;
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}
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let OutOperandList = (outs) in {
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def foo : Instruction {
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let InOperandList = (ins i32imm:$factor);
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let EncodingInfos = EncodingByHwMode<
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[ModeA, ModeB], [fooTypeEncA, fooTypeEncB]
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>;
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let AsmString = "foo $factor";
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}
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// Encoding not overridden, same namespace:
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// In the default case, this instruction is duplicated into both ModeA and
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// ModeB decoder tables.
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// In the suppressed case, this instruction appears in a single decoder table.
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def bar: Instruction {
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let InOperandList = (ins i32imm:$factor);
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let Size = 4;
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bits<32> Inst;
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bits<32> SoftFail;
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bits<8> factor;
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let Inst{31...24} = factor;
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let Inst{1...0} = 0b10;
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let AsmString = "bar $factor";
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}
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def baz : Instruction {
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let InOperandList = (ins i32imm:$factor);
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bits<32> Inst;
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let EncodingInfos = EncodingByHwMode<
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[ModeB], [fooTypeEncA]
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>;
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let AsmString = "foo $factor";
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}
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// Encoding not overridden, different namespace:
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// In the default case, this instruction is duplicated into two Alt decoder
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// tables (ModeA and ModeB).
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// In the suppressed case, this instruction appears in a single decoder table.
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def unrelated: Instruction {
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let DecoderNamespace = "Alt";
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let InOperandList = (ins i32imm:$factor);
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let Size = 4;
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bits<32> Inst;
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bits<32> SoftFail;
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bits<8> factor;
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let Inst{31...24} = factor;
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let Inst{1...0} = 0b10;
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let AsmString = "unrelated $factor";
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}
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}
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// DECODER-LABEL: DecoderTable_ModeA32
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// DECODER-DAG: decode to fooTypeEncA:foo
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// DECODER-DAG: decode to bar
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// DECODER-LABEL: DecoderTable_ModeB32
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// DECODER-DAG: decode to fooTypeEncB:foo
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// DECODER-DAG: decode to fooTypeEncA:baz
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// DECODER-DAG: decode to bar
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// DECODER-LABEL: DecoderTableAlt_ModeA32
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// DECODER-DAG: decode to unrelated
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// DECODER-LABEL: DecoderTableAlt_ModeB32
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// DECODER-DAG: decode to unrelated
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// DECODER-SUPPRESS-LABEL: DecoderTable32
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// DECODER-SUPPRESS-DAG: decode to bar
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// DECODER-SUPPRESS-LABEL: DecoderTable_ModeA32
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// DECODER-SUPPRESS-DAG: decode to fooTypeEncA:foo
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// DECODER-SUPPRESS-NOT: decode to bar
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// DECODER-SUPPRESS-LABEL: DecoderTable_ModeB32
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// DECODER-SUPPRESS-DAG: decode to fooTypeEncB:foo
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// DECODER-SUPPRESS-DAG: decode to fooTypeEncA:baz
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// DECODER-SUPPRESS-NOT: decode to bar
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// DECODER-SUPPRESS-LABEL: DecoderTableAlt32
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// DECODER-SUPPRESS-DAG: decode to unrelated
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