[RISCV] Track Linker Relaxable through Assembly Relaxation

llvm/lib/MC/MCFragment.cpp

@@ -68,6 +68,8 @@ LLVM_DUMP_METHOD void MCFragment::dump() const {
       OS << "\n  Fixup @" << F.getOffset() << " Value:";
       F.getValue()->print(OS, nullptr);
       OS << " Kind:" << F.getKind();
+      if (F.isLinkerRelaxable())
+        OS << " LinkerRelaxable";
     }
   };
 

llvm/lib/MC/MCObjectStreamer.cpp

@@ -461,11 +461,23 @@ void MCObjectStreamer::emitInstToFragment(const MCInst &Inst,
   getAssembler().getEmitter().encodeInstruction(Inst, Data, Fixups, STI);
 
   F->Kind = MCFragment::FT_Relaxable;
-  F->STI = &STI;
-  F->HasInstructions = true;
+  F->setHasInstructions(STI);
+
   F->setVarContents(Data);
-  F->setVarFixups(Fixups);
   F->setInst(Inst);
+
+  bool MarkedLinkerRelaxable = false;
+  for (auto &Fixup : Fixups) {
+    if (!Fixup.isLinkerRelaxable() || MarkedLinkerRelaxable)
+      continue;
+    MarkedLinkerRelaxable = true;
+    auto *Sec = F->getParent();
+    if (!Sec->isLinkerRelaxable())
+      Sec->setFirstLinkerRelaxable(F->getLayoutOrder());
+    F->setLinkerRelaxable();
+  }
+  F->setVarFixups(Fixups);
+
   newFragment();
 }
 

llvm/lib/MC/MCSection.cpp

@@ -39,6 +39,8 @@ LLVM_DUMP_METHOD void MCSection::dump(
   raw_ostream &OS = errs();
 
   OS << "MCSection Name:" << getName();
+  if (isLinkerRelaxable())
+    OS << " FirstLinkerRelaxable:" << firstLinkerRelaxable();
   for (auto &F : *this) {
     OS << '\n';
     F.dump();

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

@@ -819,6 +819,41 @@ void RISCVAsmBackend::maybeAddVendorReloc(const MCFragment &F,
   Asm->getWriter().recordRelocation(F, VendorFixup, VendorTarget, VendorValue);
 }
 
+static bool fixupGetsRelaxRelocation(const MCFixupKind Kind) {
+  switch (Kind) {
+  default:
+    break;
+  case RISCV::fixup_riscv_lo12_i:
+  case RISCV::fixup_riscv_lo12_s:
+  case RISCV::fixup_riscv_hi20:
+  case RISCV::fixup_riscv_pcrel_lo12_i:
+  case RISCV::fixup_riscv_pcrel_lo12_s:
+  case RISCV::fixup_riscv_pcrel_hi20:
+  case RISCV::fixup_riscv_call_plt:
+  case RISCV::fixup_riscv_qc_abs20_u:
+  case RISCV::fixup_riscv_qc_e_32:
+  case RISCV::fixup_riscv_qc_e_call_plt:
+  case ELF::R_RISCV_TPREL_LO12_I:
+  case ELF::R_RISCV_TPREL_LO12_S:
+  case ELF::R_RISCV_TPREL_HI20:
+  case ELF::R_RISCV_TPREL_ADD:
+    return true;
+  }
+  return false;
+}
+
+void RISCVAsmBackend::maybeAddRelaxReloc(const MCFragment &F,
+                                         const MCFixup &Fixup) {
+  if (!Fixup.isLinkerRelaxable() || !fixupGetsRelaxRelocation(Fixup.getKind()))
+    return;
+
+  MCFixup RelaxFixup =
+      MCFixup::create(Fixup.getOffset(), nullptr, ELF::R_RISCV_RELAX);
+  MCValue RelaxTarget = MCValue::get(nullptr);
+  uint64_t RelaxValue;
+  Asm->getWriter().recordRelocation(F, RelaxFixup, RelaxTarget, RelaxValue);
+}
+
 bool RISCVAsmBackend::addReloc(const MCFragment &F, const MCFixup &Fixup,
                                const MCValue &Target, uint64_t &FixedValue,
                                bool IsResolved) {
@@ -861,25 +896,24 @@ bool RISCVAsmBackend::addReloc(const MCFragment &F, const MCFixup &Fixup,
     return false;
   }
 
-  // If linker relaxation is enabled and supported by the current relocation,
-  // generate a relocation and then append a RELAX.
-  if (Fixup.isLinkerRelaxable())
+  // If linker relaxation is enabled and supported by the current fixup, then we
+  // always want to generate a relocation.
+  if (Fixup.isLinkerRelaxable() && fixupGetsRelaxRelocation(Fixup.getKind()))
     IsResolved = false;
+
   if (IsResolved && Fixup.isPCRel())
     IsResolved = isPCRelFixupResolved(Target.getAddSym(), F);
 
   if (!IsResolved) {
-    // Some Fixups require a vendor relocation, record it (directly) before we
+    // Some Fixups require a VENDOR relocation, record it (directly) before we
     // add the relocation.
     maybeAddVendorReloc(F, Fixup);
 
     Asm->getWriter().recordRelocation(F, Fixup, Target, FixedValue);
-  }
 
-  if (Fixup.isLinkerRelaxable()) {
-    auto FA = MCFixup::create(Fixup.getOffset(), nullptr, ELF::R_RISCV_RELAX);
-    Asm->getWriter().recordRelocation(F, FA, MCValue::get(nullptr),
-                                      FixedValueA);
+    // Some Fixups may get a RELAX relocation, record it (directly) after we
+    // add the relocation.
+    maybeAddRelaxReloc(F, Fixup);
   }
 
   return false;

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

@@ -44,6 +44,7 @@ class RISCVAsmBackend : public MCAsmBackend {
                 uint64_t &FixedValue, bool IsResolved);
 
   void maybeAddVendorReloc(const MCFragment &, const MCFixup &);
+  void maybeAddRelaxReloc(const MCFragment &, const MCFixup &);
 
   void applyFixup(const MCFragment &, const MCFixup &, const MCValue &Target,
                   uint8_t *Data, uint64_t Value, bool IsResolved) override;

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

@@ -576,8 +576,21 @@ uint64_t RISCVMCCodeEmitter::getImmOpValue(const MCInst &MI, unsigned OpNo,
          "getImmOpValue expects only expressions or immediates");
   const MCExpr *Expr = MO.getExpr();
   MCExpr::ExprKind Kind = Expr->getKind();
-  unsigned FixupKind = RISCV::fixup_riscv_invalid;
+
+  // `RelaxCandidate` must be set to `true` in two cases:
+  // - The fixup's relocation gets a R_RISCV_RELAX relocation
+  // - The underlying instruction may be relaxed to an instruction that gets a
+  //   `R_RISCV_RELAX` relocation.
+  //
+  // The actual emission of `R_RISCV_RELAX` will be handled in
+  // `RISCVAsmBackend`.
   bool RelaxCandidate = false;
+  auto AsmRelaxToLinkerRelaxableWithFeature = [&](unsigned Feature) -> void {
+    if (!STI.hasFeature(RISCV::FeatureExactAssembly) && STI.hasFeature(Feature))
+      RelaxCandidate = true;
+  };
+
+  unsigned FixupKind = RISCV::fixup_riscv_invalid;
   if (Kind == MCExpr::Specifier) {
     const auto *RVExpr = cast<MCSpecifierExpr>(Expr);
     FixupKind = RVExpr->getSpecifier();
@@ -644,18 +657,26 @@ uint64_t RISCVMCCodeEmitter::getImmOpValue(const MCInst &MI, unsigned OpNo,
     // FIXME: Sub kind binary exprs have chance of underflow.
     if (MIFrm == RISCVII::InstFormatJ) {
       FixupKind = RISCV::fixup_riscv_jal;
+      AsmRelaxToLinkerRelaxableWithFeature(RISCV::FeatureVendorXqcilb);
     } else if (MIFrm == RISCVII::InstFormatB) {
       FixupKind = RISCV::fixup_riscv_branch;
+      // This might be assembler relaxed to `b<cc>; jal` but we cannot relax
+      // the `jal` again in the assembler.
     } else if (MIFrm == RISCVII::InstFormatCJ) {
       FixupKind = RISCV::fixup_riscv_rvc_jump;
+      AsmRelaxToLinkerRelaxableWithFeature(RISCV::FeatureVendorXqcilb);
     } else if (MIFrm == RISCVII::InstFormatCB) {
       FixupKind = RISCV::fixup_riscv_rvc_branch;
+      // This might be assembler relaxed to `b<cc>; jal` but we cannot relax
+      // the `jal` again in the assembler.
     } else if (MIFrm == RISCVII::InstFormatCI) {
       FixupKind = RISCV::fixup_riscv_rvc_imm;
     } else if (MIFrm == RISCVII::InstFormatI) {
       FixupKind = RISCV::fixup_riscv_12_i;
     } else if (MIFrm == RISCVII::InstFormatQC_EB) {
       FixupKind = RISCV::fixup_riscv_qc_e_branch;
+      // This might be assembler relaxed to `qc.e.b<cc>; jal` but we cannot
+      // relax the `jal` again in the assembler.
     } else if (MIFrm == RISCVII::InstFormatQC_EAI) {
       FixupKind = RISCV::fixup_riscv_qc_e_32;
       RelaxCandidate = true;
@@ -670,9 +691,9 @@ uint64_t RISCVMCCodeEmitter::getImmOpValue(const MCInst &MI, unsigned OpNo,
   assert(FixupKind != RISCV::fixup_riscv_invalid && "Unhandled expression!");
 
   addFixup(Fixups, 0, Expr, FixupKind);
-  // If linker relaxation is enabled and supported by this relocation, set
-  // a bit so that if fixup is unresolved, a R_RISCV_RELAX relocation will be
-  // appended.
+  // If linker relaxation is enabled and supported by this relocation, set a bit
+  // so that the assembler knows the size of the instruction is not fixed/known,
+  // and the relocation will need a R_RISCV_RELAX relocation.
   if (EnableRelax && RelaxCandidate)
     Fixups.back().setLinkerRelaxable();
   ++MCNumFixups;

llvm/test/MC/RISCV/Relocations/mc-dump.s

@@ -2,12 +2,12 @@
 # RUN: llvm-mc -filetype=obj --triple=riscv64 --mattr=+relax %s -debug-only=mc-dump -o /dev/null 2>&1 | FileCheck %s
 
 #      CHECK:Sections:[
-# CHECK-NEXT:MCSection Name:.text
+# CHECK-NEXT:MCSection Name:.text FirstLinkerRelaxable:1
 # CHECK-NEXT:0 Align Size:0+0 []
 # CHECK-NEXT:  Align:4 Fill:0 FillLen:1 MaxBytesToEmit:4 Nops
 # CHECK-NEXT:  Symbol @0 .text
 # CHECK-NEXT:0 Data LinkerRelaxable Size:8 [97,00,00,00,e7,80,00,00]
-# CHECK-NEXT:  Fixup @0 Value:specifier(19,ext) Kind:4023
+# CHECK-NEXT:  Fixup @0 Value:specifier(19,ext) Kind:4023 LinkerRelaxable
 # CHECK-NEXT:  Symbol @0 $x
 # CHECK-NEXT:8 Align LinkerRelaxable Size:0+6 []
 # CHECK-NEXT:  Align:8 Fill:0 FillLen:1 MaxBytesToEmit:8 Nops

llvm/test/MC/RISCV/linker-relaxation-pr150071.s

@@ -0,0 +1,79 @@
+# RUN: llvm-mc --triple=riscv32 -mattr=+relax,+experimental-xqcilb,+experimental-xqcibi \
+# RUN:    %s -filetype=obj -o - -riscv-add-build-attributes \
+# RUN:    | llvm-objdump -dr -M no-aliases - \
+# RUN:    | FileCheck %s
+
+.global foo
+
+# CHECK-LABEL: <branch_over_relaxable>:
+branch_over_relaxable:
+  jal x1, foo
+# CHECK: qc.e.jal 0x0 <branch_over_relaxable>
+# CHECK-NEXT: R_RISCV_VENDOR QUALCOMM
+# CHECK-NEXT: R_RISCV_CUSTOM195 foo
+# CHECK-NEXT: R_RISCV_RELAX *ABS*
+  bne a0, a1, branch_over_relaxable
+# CHECK-NEXT: bne a0, a1, 0x6 <branch_over_relaxable+0x6>
+# CHECK-NEXT: R_RISCV_BRANCH branch_over_relaxable
+# CHECK-NOT: R_RISCV_RELAX
+  qc.e.bnei a0, 0x21, branch_over_relaxable
+# CHECK-NEXT: qc.e.bnei a0, 0x21, 0xa <branch_over_relaxable+0xa>
+# CHECK-NEXT: R_RISCV_VENDOR QUALCOMM
+# CHECK-NEXT: R_RISCV_CUSTOM193 branch_over_relaxable
+# CHECK-NOT: R_RISCV_RELAX
+  ret
+# CHECK-NEXT: c.jr ra
+
+# CHECK-LABEL: <short_jump_over_fixed>:
+short_jump_over_fixed:
+  nop
+# CHECK: c.nop
+  j short_jump_over_fixed
+# CHECK-NEXT: c.j 0x12 <short_jump_over_fixed>
+# CHECK-NOT: R_RISCV_RVC_JUMP
+# CHECK-NOT: R_RISCV_RELAX
+  ret
+# CHECK-NEXT: c.jr ra
+
+# CHECK-LABEL: <short_jump_over_relaxable>:
+short_jump_over_relaxable:
+  call foo
+# CHECK: auipc ra, 0x0
+# CHECK-NEXT: R_RISCV_CALL_PLT foo
+# CHECK-NEXT: R_RISCV_RELAX *ABS*
+# CHECK-NEXT: jalr ra, 0x0(ra) <short_jump_over_relaxable>
+  j short_jump_over_relaxable
+# CHECK-NEXT: c.j 0x20 <short_jump_over_relaxable+0x8>
+# CHECK-NEXT: R_RISCV_RVC_JUMP short_jump_over_relaxable
+# CHECK-NOT: R_RISCV_RELAX
+  ret
+# CHECK-NEXT: c.jr ra
+
+# CHECK-LABEL: <mid_jump_over_fixed>:
+mid_jump_over_fixed:
+  nop
+# CHECK: c.nop
+  .space 0x1000
+# CHECK-NEXT: ...
+  j mid_jump_over_fixed
+# CHECK-NEXT: jal zero, 0x24 <mid_jump_over_fixed>
+# CHECK-NOT: R_RISCV_JAL
+# CHECK-NOT: R_RISCV_RELAX
+  ret
+# CHECK-NEXT: c.jr ra
+
+# CHECK-LABEL: <mid_jump_over_relaxable>:
+mid_jump_over_relaxable:
+  call foo
+# CHECK: auipc ra, 0x0
+# CHECK-NEXT: R_RISCV_CALL_PLT foo
+# CHECK-NEXT: R_RISCV_RELAX *ABS*
+# CHECK-NEXT: jalr ra, 0x0(ra) <mid_jump_over_relaxable>
+  .space 0x1000
+# CHECK-NEXT: ...
+  j mid_jump_over_relaxable
+# CHECK-NEXT: jal zero, 0x2034 <mid_jump_over_relaxable+0x1008>
+# CHECK-NEXT: R_RISCV_JAL mid_jump_over_relaxable
+# CHECK-NOT: R_RISCV_RELAX
+  ret
+# CHECK-NEXT: c.jr ra