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Add support for RV32E in RegFilePlugin 

The RV32E extension removes registers x16-x31 from the ISA.  This
is useful when compiling with -mem2reg to save on BRAMs.  On iCE40
HX8K this option saves 1285 LC:s, which also improves the routing
situation, when using -mem2reg.

Note that the illegal instruction exception required by the RV32E
specification for accesses to registers x16-x31 is not implemented.
This commit is contained in:
Marcus Comstedt 2020-09-06 17:05:31 +02:00
parent 2942d0652a
commit 8e466dd13c
1 changed files with 11 additions and 4 deletions
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15
src/main/scala/vexriscv/plugin/RegFilePlugin.scala
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@ -18,6 +18,7 @@ class RegFilePlugin(regFileReadyKind : RegFileReadKind,
writeRfInMemoryStage : Boolean = false, writeRfInMemoryStage : Boolean = false,
readInExecute : Boolean = false, readInExecute : Boolean = false,
syncUpdateOnStall : Boolean = true, syncUpdateOnStall : Boolean = true,
rv32e : Boolean = false,
withShadow : Boolean = false //shadow registers aren't transition hazard free withShadow : Boolean = false //shadow registers aren't transition hazard free
) extends Plugin[VexRiscv] with RegFileService{ ) extends Plugin[VexRiscv] with RegFileService{
import Riscv._ import Riscv._
@ -39,8 +40,11 @@ class RegFilePlugin(regFileReadyKind : RegFileReadKind,
val readStage = if(readInExecute) execute else decode val readStage = if(readInExecute) execute else decode
val writeStage = if(writeRfInMemoryStage) memory else stages.last val writeStage = if(writeRfInMemoryStage) memory else stages.last
val numRegisters = if(rv32e) 16 else 32
def clipRange(that : Range) = if(rv32e) that.tail else that
val global = pipeline plug new Area{ val global = pipeline plug new Area{
val regFileSize = if(withShadow) 64 else 32 val regFileSize = if(withShadow) numRegisters * 2 else numRegisters
val regFile = Mem(Bits(32 bits),regFileSize) addAttribute(Verilator.public) val regFile = Mem(Bits(32 bits),regFileSize) addAttribute(Verilator.public)
if(zeroBoot) regFile.init(List.fill(regFileSize)(B(0, 32 bits))) if(zeroBoot) regFile.init(List.fill(regFileSize)(B(0, 32 bits)))
@ -59,6 +63,9 @@ class RegFilePlugin(regFileReadyKind : RegFileReadKind,
when(decode.input(INSTRUCTION)(rdRange) === 0) { when(decode.input(INSTRUCTION)(rdRange) === 0) {
decode.input(REGFILE_WRITE_VALID) := False decode.input(REGFILE_WRITE_VALID) := False
} }
if(rv32e) when(decode.input(INSTRUCTION)(rdRange.head)) {
decode.input(REGFILE_WRITE_VALID) := False
}
//Read register file //Read register file
readStage plug new Area{ readStage plug new Area{
@ -72,8 +79,8 @@ class RegFilePlugin(regFileReadyKind : RegFileReadKind,
} }
def shadowPrefix(that : Bits) = if(withShadow) global.shadow.read ## that else that def shadowPrefix(that : Bits) = if(withShadow) global.shadow.read ## that else that
val regFileReadAddress1 = U(shadowPrefix(srcInstruction(Riscv.rs1Range))) val regFileReadAddress1 = U(shadowPrefix(srcInstruction(clipRange(Riscv.rs1Range))))
val regFileReadAddress2 = U(shadowPrefix(srcInstruction(Riscv.rs2Range))) val regFileReadAddress2 = U(shadowPrefix(srcInstruction(clipRange(Riscv.rs2Range))))
val (rs1Data,rs2Data) = regFileReadyKind match{ val (rs1Data,rs2Data) = regFileReadyKind match{
case `ASYNC` => (global.regFile.readAsync(regFileReadAddress1),global.regFile.readAsync(regFileReadAddress2)) case `ASYNC` => (global.regFile.readAsync(regFileReadAddress1),global.regFile.readAsync(regFileReadAddress2))
@ -93,7 +100,7 @@ class RegFilePlugin(regFileReadyKind : RegFileReadKind,
def shadowPrefix(that : Bits) = if(withShadow) global.shadow.write ## that else that def shadowPrefix(that : Bits) = if(withShadow) global.shadow.write ## that else that
val regFileWrite = global.regFile.writePort.addAttribute(Verilator.public).setName("lastStageRegFileWrite") val regFileWrite = global.regFile.writePort.addAttribute(Verilator.public).setName("lastStageRegFileWrite")
regFileWrite.valid := output(REGFILE_WRITE_VALID) && arbitration.isFiring regFileWrite.valid := output(REGFILE_WRITE_VALID) && arbitration.isFiring
regFileWrite.address := U(shadowPrefix(output(INSTRUCTION)(rdRange))) regFileWrite.address := U(shadowPrefix(output(INSTRUCTION)(clipRange(rdRange))))
regFileWrite.data := output(REGFILE_WRITE_DATA) regFileWrite.data := output(REGFILE_WRITE_DATA)
//Ensure no boot glitches modify X0 //Ensure no boot glitches modify X0