Index: llvm/include/llvm/MC/MCFragment.h =================================================================== --- llvm/include/llvm/MC/MCFragment.h +++ llvm/include/llvm/MC/MCFragment.h @@ -34,6 +34,7 @@ public: enum FragmentType : uint8_t { FT_Align, + FT_NeverAlign, FT_Data, FT_CompactEncodedInst, FT_Fill, @@ -333,6 +334,21 @@ } }; +class MCNeverAlignFragment : public MCFragment { + /// The alignment the end of the next fragment should avoid. + unsigned Alignment; + +public: + MCNeverAlignFragment(unsigned Alignment, MCSection *Sec = nullptr) + : MCFragment(FT_NeverAlign, false, Sec), Alignment(Alignment) {} + + unsigned getAlignment() const { return Alignment; } + + static bool classof(const MCFragment *F) { + return F->getKind() == MCFragment::FT_NeverAlign; + } +}; + class MCFillFragment : public MCFragment { uint8_t ValueSize; /// Value to use for filling bytes. Index: llvm/include/llvm/MC/MCObjectStreamer.h =================================================================== --- llvm/include/llvm/MC/MCObjectStreamer.h +++ llvm/include/llvm/MC/MCObjectStreamer.h @@ -139,6 +139,7 @@ unsigned MaxBytesToEmit = 0) override; void emitCodeAlignment(unsigned ByteAlignment, unsigned MaxBytesToEmit = 0) override; + void emitNeverAlignCodeAtEnd(unsigned ByteAlignment) override; void emitValueToOffset(const MCExpr *Offset, unsigned char Value, SMLoc Loc) override; void emitDwarfLocDirective(unsigned FileNo, unsigned Line, unsigned Column, Index: llvm/include/llvm/MC/MCStreamer.h =================================================================== --- llvm/include/llvm/MC/MCStreamer.h +++ llvm/include/llvm/MC/MCStreamer.h @@ -834,6 +834,10 @@ virtual void emitCodeAlignment(unsigned ByteAlignment, unsigned MaxBytesToEmit = 0); + /// If the end of the following fragment ever gets aligned to + /// \p ByteAlignment, emit a single nop to break this alignment. + virtual void emitNeverAlignCodeAtEnd(unsigned ByteAlignment); + /// Emit some number of copies of \p Value until the byte offset \p /// Offset is reached. /// Index: llvm/lib/MC/MCAssembler.cpp =================================================================== --- llvm/lib/MC/MCAssembler.cpp +++ llvm/lib/MC/MCAssembler.cpp @@ -346,6 +346,27 @@ return Size; } + case MCFragment::FT_NeverAlign: { + const MCNeverAlignFragment &NAF = cast(F); + const MCFragment *NF = F.getNextNode(); + uint64_t Offset = Layout.getFragmentOffset(&NAF); + size_t NextFragSize = 0; + if (const auto *NextFrag = dyn_cast(NF)) { + NextFragSize = NextFrag->getContents().size(); + } else if (const auto *NextFrag = dyn_cast(NF)) { + NextFragSize = NextFrag->getContents().size(); + } else { + llvm_unreachable("Didn't find the expected fragment after NeverAlign"); + } + // Check if the next fragment ends at the alignment we want to avoid. + if ((Offset + NextFragSize) % NAF.getAlignment() == 0) { + // Avoid this alignment by introducing minimum nop. + assert(getBackend().getMinimumNopSize() != NAF.getAlignment()); + return getBackend().getMinimumNopSize(); + } + return 0; + } + case MCFragment::FT_Org: { const MCOrgFragment &OF = cast(F); MCValue Value; @@ -569,6 +590,13 @@ break; } + case MCFragment::FT_NeverAlign: { + if (!Asm.getBackend().writeNopData(OS, FragmentSize)) + report_fatal_error("unable to write nop sequence of " + + Twine(FragmentSize) + " bytes"); + break; + } + case MCFragment::FT_Data: ++stats::EmittedDataFragments; OS << cast(F).getContents(); Index: llvm/lib/MC/MCFragment.cpp =================================================================== --- llvm/lib/MC/MCFragment.cpp +++ llvm/lib/MC/MCFragment.cpp @@ -270,6 +270,9 @@ case FT_Align: delete cast(this); return; + case FT_NeverAlign: + delete cast(this); + return; case FT_Data: delete cast(this); return; @@ -338,6 +341,9 @@ OS << "<"; switch (getKind()) { case MCFragment::FT_Align: OS << "MCAlignFragment"; break; + case MCFragment::FT_NeverAlign: + OS << "MCNeverAlignFragment"; + break; case MCFragment::FT_Data: OS << "MCDataFragment"; break; case MCFragment::FT_CompactEncodedInst: OS << "MCCompactEncodedInstFragment"; break; @@ -377,6 +383,12 @@ << " MaxBytesToEmit:" << AF->getMaxBytesToEmit() << ">"; break; } + case MCFragment::FT_NeverAlign: { + const MCNeverAlignFragment *NAF = cast(this); + OS << "\n "; + OS << " Alignment:" << NAF->getAlignment() << ">"; + break; + } case MCFragment::FT_Data: { const auto *DF = cast(this); OS << "\n "; Index: llvm/lib/MC/MCObjectStreamer.cpp =================================================================== --- llvm/lib/MC/MCObjectStreamer.cpp +++ llvm/lib/MC/MCObjectStreamer.cpp @@ -619,6 +619,10 @@ cast(getCurrentFragment())->setEmitNops(true); } +void MCObjectStreamer::emitNeverAlignCodeAtEnd(unsigned ByteAlignment) { + insert(new MCNeverAlignFragment(ByteAlignment)); +} + void MCObjectStreamer::emitValueToOffset(const MCExpr *Offset, unsigned char Value, SMLoc Loc) { Index: llvm/lib/MC/MCStreamer.cpp =================================================================== --- llvm/lib/MC/MCStreamer.cpp +++ llvm/lib/MC/MCStreamer.cpp @@ -1184,6 +1184,7 @@ unsigned MaxBytesToEmit) {} void MCStreamer::emitCodeAlignment(unsigned ByteAlignment, unsigned MaxBytesToEmit) {} +void MCStreamer::emitNeverAlignCodeAtEnd(unsigned ByteAlignment) {} void MCStreamer::emitValueToOffset(const MCExpr *Offset, unsigned char Value, SMLoc Loc) {} void MCStreamer::emitBundleAlignMode(unsigned AlignPow2) {} Index: llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp =================================================================== --- llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp +++ llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp @@ -1128,6 +1128,7 @@ bool parseDirectiveArch(); bool parseDirectiveNops(SMLoc L); bool parseDirectiveEven(SMLoc L); + bool parseDirectiveAvoidEndAlign(SMLoc L); bool ParseDirectiveCode(StringRef IDVal, SMLoc L); /// CodeView FPO data directives. @@ -4631,6 +4632,8 @@ return false; } else if (IDVal == ".nops") return parseDirectiveNops(DirectiveID.getLoc()); + else if (IDVal == ".avoid_end_align") + return parseDirectiveAvoidEndAlign(DirectiveID.getLoc()); else if (IDVal == ".even") return parseDirectiveEven(DirectiveID.getLoc()); else if (IDVal == ".cv_fpo_proc") @@ -4726,6 +4729,33 @@ return false; } +/// parseDirectiveAvoidEndAlign +/// ::= .avoid_end_align alignment +bool X86AsmParser::parseDirectiveAvoidEndAlign(SMLoc L) { + int64_t Alignment = 0; + SMLoc AlignmentLoc; + const MCSubtargetInfo STI = getSTI(); + AlignmentLoc = getTok().getLoc(); + if (getParser().checkForValidSection() || + getParser().parseAbsoluteExpression(Alignment)) + return true; + + if (getParser().parseToken( + AsmToken::EndOfStatement, + "unexpected token in '.avoid_end_align' directive")) + return true; + + if (Alignment <= 0) { + Error(AlignmentLoc, "'.avoid_end_align' directive with non-positive size"); + return false; + } + + /// Emit NeverAlign + getParser().getStreamer().emitNeverAlignCodeAtEnd(Alignment); + + return false; +} + /// ParseDirectiveCode /// ::= .code16 | .code32 | .code64 bool X86AsmParser::ParseDirectiveCode(StringRef IDVal, SMLoc L) { Index: llvm/test/MC/X86/x86_64-directive-avoid_end_align.s =================================================================== --- /dev/null +++ llvm/test/MC/X86/x86_64-directive-avoid_end_align.s @@ -0,0 +1,19 @@ +# RUN: llvm-mc -triple=x86_64 %s -filetype=obj | llvm-objdump --no-show-raw-insn -d - | FileCheck %s + +# avoid_end_align has no effect since test doesn't end at alignment boundary: +.avoid_end_align 64 +# CHECK-NOT: nop + testl %eax, %eax +# CHECK: testl %eax, %eax + je .LBB0 + +.fill 58, 1, 0x00 +# avoid_end_align inserts nop because `test` would end at alignment boundary: +.avoid_end_align 64 +# CHECK: 3e: nop + testl %eax, %eax +# CHECK-NEXT: 3f: testl %eax, %eax + je .LBB0 +# CHECK-NEXT: 41: je +.LBB0: + retq Index: llvm/test/MC/X86/x86_64-directive-avoid_end_align_errors.s =================================================================== --- /dev/null +++ llvm/test/MC/X86/x86_64-directive-avoid_end_align_errors.s @@ -0,0 +1,11 @@ +# RUN: not llvm-mc -triple x86_64-unknown-unknown %s 2> %t.err +# RUN: FileCheck < %t.err %s + +.avoid_end_align +# CHECK: unknown token in expression +.avoid_end_align x +# CHECK: expected absolute expression +.avoid_end_align 0 +# CHECK: '.avoid_end_align' directive with non-positive size +.avoid_end_align 64, 0 +# CHECK: unexpected token in '.avoid_end_align' directive