[RISCV] Support .option {no}exact

llvm/lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp

@@ -3173,6 +3173,26 @@ bool RISCVAsmParser::parseDirectiveOption() {
     return false;
   }
 
+  if (Option == "exact") {
+    if (Parser.parseEOL())
+      return true;
+
+    getTargetStreamer().emitDirectiveOptionExact();
+    setFeatureBits(RISCV::FeatureExactAssembly, "exact-asm");
+    clearFeatureBits(RISCV::FeatureRelax, "relax");
+    return false;
+  }
+
+  if (Option == "noexact") {
+    if (Parser.parseEOL())
+      return true;
+
+    getTargetStreamer().emitDirectiveOptionNoExact();
+    clearFeatureBits(RISCV::FeatureExactAssembly, "exact-asm");
+    setFeatureBits(RISCV::FeatureRelax, "relax");
+    return false;
+  }
+
   if (Option == "rvc") {
     if (Parser.parseEOL())
       return true;
@@ -3229,9 +3249,10 @@ bool RISCVAsmParser::parseDirectiveOption() {
   }
 
   // Unknown option.
-  Warning(Parser.getTok().getLoc(), "unknown option, expected 'push', 'pop', "
-                                    "'rvc', 'norvc', 'arch', 'relax' or "
-                                    "'norelax'");
+  Warning(Parser.getTok().getLoc(),
+          "unknown option, expected 'push', 'pop', "
+          "'rvc', 'norvc', 'arch', 'relax', 'norelax', "
+          "'exact' or 'noexact'");
   Parser.eatToEndOfStatement();
   return false;
 }
@@ -3441,10 +3462,13 @@ bool RISCVAsmParser::parseDirectiveVariantCC() {
 
 void RISCVAsmParser::emitToStreamer(MCStreamer &S, const MCInst &Inst) {
   MCInst CInst;
-  bool Res = RISCVRVC::compress(CInst, Inst, getSTI());
+  bool Res = false;
+  const MCSubtargetInfo &STI = getSTI();
+  if (!STI.hasFeature(RISCV::FeatureExactAssembly))
+    Res = RISCVRVC::compress(CInst, Inst, STI);
   if (Res)
     ++RISCVNumInstrsCompressed;
-  S.emitInstruction((Res ? CInst : Inst), getSTI());
+  S.emitInstruction((Res ? CInst : Inst), STI);
 }
 
 void RISCVAsmParser::emitLoadImm(MCRegister DestReg, int64_t Value,

llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp

@@ -8,6 +8,7 @@
 
 #include "RISCVAsmBackend.h"
 #include "RISCVMCExpr.h"
+#include "RISCVMCTargetDesc.h"
 #include "llvm/ADT/APInt.h"
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/MC/MCAssembler.h"
@@ -171,8 +172,39 @@ bool RISCVAsmBackend::fixupNeedsRelaxationAdvanced(
   }
 }
 
+// Given a compressed control flow instruction this function returns
+// the expanded instruction.
+static unsigned getRelaxedOpcode(unsigned Op) {
+  switch (Op) {
+  default:
+    return Op;
+  case RISCV::C_BEQZ:
+    return RISCV::BEQ;
+  case RISCV::C_BNEZ:
+    return RISCV::BNE;
+  case RISCV::C_J:
+  case RISCV::C_JAL: // fall through.
+    return RISCV::JAL;
+  case RISCV::BEQ:
+    return RISCV::PseudoLongBEQ;
+  case RISCV::BNE:
+    return RISCV::PseudoLongBNE;
+  case RISCV::BLT:
+    return RISCV::PseudoLongBLT;
+  case RISCV::BGE:
+    return RISCV::PseudoLongBGE;
+  case RISCV::BLTU:
+    return RISCV::PseudoLongBLTU;
+  case RISCV::BGEU:
+    return RISCV::PseudoLongBGEU;
+  }
+}
+
 void RISCVAsmBackend::relaxInstruction(MCInst &Inst,
                                        const MCSubtargetInfo &STI) const {
+  if (STI.hasFeature(RISCV::FeatureExactAssembly))
+    return;
+
   MCInst Res;
   switch (Inst.getOpcode()) {
   default:
@@ -341,36 +373,14 @@ std::pair<bool, bool> RISCVAsmBackend::relaxLEB128(const MCAssembler &Asm,
   return std::make_pair(Expr.evaluateKnownAbsolute(Value, Asm), false);
 }
 
-// Given a compressed control flow instruction this function returns
-// the expanded instruction.
-unsigned RISCVAsmBackend::getRelaxedOpcode(unsigned Op) const {
-  switch (Op) {
-  default:
-    return Op;
-  case RISCV::C_BEQZ:
-    return RISCV::BEQ;
-  case RISCV::C_BNEZ:
-    return RISCV::BNE;
-  case RISCV::C_J:
-  case RISCV::C_JAL: // fall through.
-    return RISCV::JAL;
-  case RISCV::BEQ:
-    return RISCV::PseudoLongBEQ;
-  case RISCV::BNE:
-    return RISCV::PseudoLongBNE;
-  case RISCV::BLT:
-    return RISCV::PseudoLongBLT;
-  case RISCV::BGE:
-    return RISCV::PseudoLongBGE;
-  case RISCV::BLTU:
-    return RISCV::PseudoLongBLTU;
-  case RISCV::BGEU:
-    return RISCV::PseudoLongBGEU;
-  }
-}
-
 bool RISCVAsmBackend::mayNeedRelaxation(const MCInst &Inst,
                                         const MCSubtargetInfo &STI) const {
+  // This function has access to two STIs, the member of the AsmBackend, and the
+  // one passed as an argument. The latter is more specific, so we query it for
+  // specific features.
+  if (STI.hasFeature(RISCV::FeatureExactAssembly))
+    return false;
+
   return getRelaxedOpcode(Inst.getOpcode()) != Inst.getOpcode();
 }
 

llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h

@@ -81,7 +81,6 @@ class RISCVAsmBackend : public MCAsmBackend {
 
   bool mayNeedRelaxation(const MCInst &Inst,
                          const MCSubtargetInfo &STI) const override;
-  unsigned getRelaxedOpcode(unsigned Op) const;
 
   void relaxInstruction(MCInst &Inst,
                         const MCSubtargetInfo &STI) const override;

llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp

@@ -53,6 +53,8 @@ void RISCVTargetELFStreamer::emitDirectiveOptionPIC() {}
 void RISCVTargetELFStreamer::emitDirectiveOptionNoPIC() {}
 void RISCVTargetELFStreamer::emitDirectiveOptionRVC() {}
 void RISCVTargetELFStreamer::emitDirectiveOptionNoRVC() {}
+void RISCVTargetELFStreamer::emitDirectiveOptionExact() {}
+void RISCVTargetELFStreamer::emitDirectiveOptionNoExact() {}
 void RISCVTargetELFStreamer::emitDirectiveOptionRelax() {}
 void RISCVTargetELFStreamer::emitDirectiveOptionNoRelax() {}
 

llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.h

@@ -62,6 +62,8 @@ class RISCVTargetELFStreamer : public RISCVTargetStreamer {
   void emitDirectiveOptionNoPIC() override;
   void emitDirectiveOptionRVC() override;
   void emitDirectiveOptionNoRVC() override;
+  void emitDirectiveOptionExact() override;
+  void emitDirectiveOptionNoExact() override;
   void emitDirectiveOptionRelax() override;
   void emitDirectiveOptionNoRelax() override;
   void emitDirectiveVariantCC(MCSymbol &Symbol) override;

llvm/lib/Target/RISCV/MCTargetDesc/RISCVTargetStreamer.cpp

@@ -39,6 +39,8 @@ void RISCVTargetStreamer::emitDirectiveOptionPIC() {}
 void RISCVTargetStreamer::emitDirectiveOptionNoPIC() {}
 void RISCVTargetStreamer::emitDirectiveOptionRVC() {}
 void RISCVTargetStreamer::emitDirectiveOptionNoRVC() {}
+void RISCVTargetStreamer::emitDirectiveOptionExact() {}
+void RISCVTargetStreamer::emitDirectiveOptionNoExact() {}
 void RISCVTargetStreamer::emitDirectiveOptionRelax() {}
 void RISCVTargetStreamer::emitDirectiveOptionNoRelax() {}
 void RISCVTargetStreamer::emitDirectiveOptionArch(
@@ -125,6 +127,14 @@ void RISCVTargetAsmStreamer::emitDirectiveOptionNoRVC() {
   OS << "\t.option\tnorvc\n";
 }
 
+void RISCVTargetAsmStreamer::emitDirectiveOptionExact() {
+  OS << "\t.option\texact\n";
+}
+
+void RISCVTargetAsmStreamer::emitDirectiveOptionNoExact() {
+  OS << "\t.option\tnoexact\n";
+}
+
 void RISCVTargetAsmStreamer::emitDirectiveOptionRelax() {
   OS << "\t.option\trelax\n";
 }

llvm/lib/Target/RISCV/MCTargetDesc/RISCVTargetStreamer.h

@@ -47,6 +47,8 @@ class RISCVTargetStreamer : public MCTargetStreamer {
   virtual void emitDirectiveOptionNoPIC();
   virtual void emitDirectiveOptionRVC();
   virtual void emitDirectiveOptionNoRVC();
+  virtual void emitDirectiveOptionExact();
+  virtual void emitDirectiveOptionNoExact();
   virtual void emitDirectiveOptionRelax();
   virtual void emitDirectiveOptionNoRelax();
   virtual void emitDirectiveOptionArch(ArrayRef<RISCVOptionArchArg> Args);
@@ -84,6 +86,8 @@ class RISCVTargetAsmStreamer : public RISCVTargetStreamer {
   void emitDirectiveOptionNoPIC() override;
   void emitDirectiveOptionRVC() override;
   void emitDirectiveOptionNoRVC() override;
+  void emitDirectiveOptionExact() override;
+  void emitDirectiveOptionNoExact() override;
   void emitDirectiveOptionRelax() override;
   void emitDirectiveOptionNoRelax() override;
   void emitDirectiveOptionArch(ArrayRef<RISCVOptionArchArg> Args) override;

llvm/lib/Target/RISCV/RISCVFeatures.td

@@ -1470,6 +1470,10 @@ def FeatureRelax
     : SubtargetFeature<"relax", "EnableLinkerRelax", "true",
                        "Enable Linker relaxation.">;
 
+def FeatureExactAssembly
+    : SubtargetFeature<"exact-asm", "EnableExactAssembly", "true",
+                       "Enable Exact Assembly (Disables Compression and Relaxation)">;
+
 foreach i = {1-31} in
   def FeatureReserveX#i :
       SubtargetFeature<"reserve-x"#i, "UserReservedRegister[RISCV::X"#i#"]",

llvm/test/MC/RISCV/option-exact-compression.s

@@ -0,0 +1,72 @@
+# RUN: llvm-mc -triple riscv32 -show-encoding -mattr=+c %s \
+# RUN:   | FileCheck -check-prefixes=CHECK,CHECK-ALIAS %s
+# RUN: llvm-mc -triple riscv32 -show-encoding -mattr=+c \
+# RUN:   -M no-aliases %s | FileCheck -check-prefixes=CHECK,CHECK-INST %s
+# RUN: llvm-mc -triple riscv32 -filetype=obj -mattr=+c %s \
+# RUN:   | llvm-objdump  --triple=riscv32 --mattr=+c --no-print-imm-hex -d - \
+# RUN:   | FileCheck -check-prefixes=CHECK-BYTES,CHECK-ALIAS %s
+# RUN: llvm-mc -triple riscv32 -filetype=obj -mattr=+c %s \
+# RUN:   | llvm-objdump  --triple=riscv32 --mattr=+c --no-print-imm-hex -d -M no-aliases - \
+# RUN:   | FileCheck -check-prefixes=CHECK-BYTES,CHECK-INST %s
+
+# RUN: llvm-mc -triple riscv64 -show-encoding -mattr=+c %s \
+# RUN:   | FileCheck -check-prefixes=CHECK-ALIAS %s
+# RUN: llvm-mc -triple riscv64 -show-encoding -mattr=+c \
+# RUN:   -M no-aliases %s | FileCheck -check-prefixes=CHECK-INST %s
+# RUN: llvm-mc -triple riscv64 -filetype=obj -mattr=+c %s \
+# RUN:   | llvm-objdump  --triple=riscv64 --mattr=+c --no-print-imm-hex -d - \
+# RUN:   | FileCheck -check-prefixes=CHECK-BYTES,CHECK-ALIAS %s
+# RUN: llvm-mc -triple riscv64 -filetype=obj -mattr=+c %s \
+# RUN:   | llvm-objdump  --triple=riscv64 --mattr=+c --no-print-imm-hex -d -M no-aliases - \
+# RUN:   | FileCheck -check-prefixes=CHECK-BYTES,CHECK-INST %s
+
+
+## `.option exact` disables a variety of assembler behaviour:
+## - automatic compression
+## - branch relaxation (of short branches to longer equivalent sequences)
+## - linker relaxation (emitting R_RISCV_RELAX)
+## `.option noexact` enables these behaviours again. It is also the default.
+
+## This test only checks the automatic compression part of this behaviour.
+
+# CHECK-BYTES: 4108
+# CHECK-INST: c.lw a0, 0(a0)
+# CHECK-ALIAS: lw a0, 0(a0)
+# CHECK: # encoding: [0x08,0x41]
+lw a0, 0(a0)
+
+# CHECK-BYTES: 4108
+# CHECK-INST: c.lw a0, 0(a0)
+# CHECK-ALIAS: lw a0, 0(a0)
+# CHECK: # encoding: [0x08,0x41]
+c.lw a0, 0(a0)
+
+# CHECK: .option exact
+.option exact
+
+# CHECK-BYTES: 00052503
+# CHECK-INST: lw a0, 0(a0)
+# CHECK-ALIAS: lw a0, 0(a0)
+# CHECK: # encoding: [0x03,0x25,0x05,0x00]
+lw a0, 0(a0)
+
+# CHECK-BYTES: 4108
+# CHECK-INST: c.lw a0, 0(a0)
+# CHECK-ALIAS: lw a0, 0(a0)
+# CHECK: # encoding: [0x08,0x41]
+c.lw a0, 0(a0)
+
+# CHECK: .option noexact
+.option noexact
+
+# CHECK-BYTES: 4108
+# CHECK-INST: c.lw a0, 0(a0)
+# CHECK-ALIAS: lw a0, 0(a0)
+# CHECK: # encoding: [0x08,0x41]
+lw a0, 0(a0)
+
+# CHECK-BYTES: 4108
+# CHECK-INST: c.lw a0, 0(a0)
+# CHECK-ALIAS: lw a0, 0(a0)
+# CHECK: # encoding: [0x08,0x41]
+c.lw a0, 0(a0)

llvm/test/MC/RISCV/option-exact-inlineasm.ll

@@ -0,0 +1,21 @@
+; RUN: llc -mtriple=riscv32 -mattr=+relax,+c %s --filetype=obj -o - \
+; RUN:  | llvm-objdump --triple=riscv32 --mattr=+c -M no-aliases -dr - \
+; RUN:  | FileCheck %s
+
+define i32 @foo(ptr noundef %f) nounwind {
+; CHECK-LABEL: <foo>:
+; CHECK:      auipc ra, 0x0
+; CHECK-NEXT:     R_RISCV_CALL_PLT undefined
+; CHECK-NEXT: jalr ra, 0x0(ra)
+; CHECK-NEXT: lw a0, 0x0(a0)
+; CHECK-NEXT: c.jr ra
+
+entry:
+  %0 = tail call i32 asm sideeffect "
+  .option exact
+  call undefined@plt
+  lw $0, ($1)
+  .option noexact", "=^cr,^cr"(ptr %f)
+  ret i32 %0
+}
+