[PATCH] D28688: MC: Introduce the ABS8 symbol modifier.

[Rafael Avila de Espindola via llvm-commits]

Why is this needed? Why not just use

cmpb $foo, %al

Cheers,
Rafael

Peter Collingbourne via Phabricator via llvm-commits
<llvm-commits at lists.llvm.org> writes:

> pcc created this revision.
> pcc added reviewers: RKSimon, craig.topper, majnemer.
> pcc added a subscriber: llvm-commits.
>
> @ABS8 can be applied to symbols which appear as immediate operands to
> instructions that have a 8-bit immediate form for that operand. It causes
> the assembler to use the 8-bit form and an 8-bit relocation (e.g. R_386_8
> or R_X86_64_8) for the symbol.
>
>
> https://reviews.llvm.org/D28688
>
> Files:
>   llvm/docs/Extensions.rst
>   llvm/include/llvm/MC/MCExpr.h
>   llvm/lib/MC/MCAssembler.cpp
>   llvm/lib/MC/MCExpr.cpp
>   llvm/lib/Target/X86/MCTargetDesc/X86ELFObjectWriter.cpp
>   llvm/test/MC/X86/abs8.s
>
> Index: llvm/test/MC/X86/abs8.s
> ===================================================================
> --- /dev/null
> +++ llvm/test/MC/X86/abs8.s
> @@ -0,0 +1,8 @@
> +// RUN: llvm-mc -filetype=obj %s -o - -triple i686-pc-linux | llvm-objdump -d -r - | FileCheck --check-prefix=32 %s
> +// RUN: llvm-mc -filetype=obj %s -o - -triple x86_64-pc-linux | llvm-objdump -d -r - | FileCheck --check-prefix=64 %s
> +
> +// 32: 0: 83 ff 00  cmpl $0, %edi
> +// 32:   00000002:  R_386_8 foo
> +// 64: 0: 83 ff 00  cmpl $0, %edi
> +// 64:  0000000000000002:  R_X86_64_8 foo+0
> +cmp $foo at ABS8, %edi
> Index: llvm/lib/Target/X86/MCTargetDesc/X86ELFObjectWriter.cpp
> ===================================================================
> --- llvm/lib/Target/X86/MCTargetDesc/X86ELFObjectWriter.cpp
> +++ llvm/lib/Target/X86/MCTargetDesc/X86ELFObjectWriter.cpp
> @@ -96,6 +96,7 @@
>    default:
>      llvm_unreachable("Unimplemented");
>    case MCSymbolRefExpr::VK_None:
> +  case MCSymbolRefExpr::VK_ABS8:
>      switch (Type) {
>      case RT64_64:
>        return IsPCRel ? ELF::R_X86_64_PC64 : ELF::R_X86_64_64;
> @@ -219,6 +220,7 @@
>    default:
>      llvm_unreachable("Unimplemented");
>    case MCSymbolRefExpr::VK_None:
> +  case MCSymbolRefExpr::VK_ABS8:
>      switch (Type) {
>      case RT32_32:
>        return IsPCRel ? ELF::R_386_PC32 : ELF::R_386_32;
> Index: llvm/lib/MC/MCExpr.cpp
> ===================================================================
> --- llvm/lib/MC/MCExpr.cpp
> +++ llvm/lib/MC/MCExpr.cpp
> @@ -205,6 +205,7 @@
>    case VK_SECREL: return "SECREL32";
>    case VK_SIZE: return "SIZE";
>    case VK_WEAKREF: return "WEAKREF";
> +  case VK_ABS8: return "ABS8";
>    case VK_ARM_NONE: return "none";
>    case VK_ARM_GOT_PREL: return "GOT_PREL";
>    case VK_ARM_TARGET1: return "target1";
> @@ -314,6 +315,7 @@
>      .Case("imgrel", VK_COFF_IMGREL32)
>      .Case("secrel32", VK_SECREL)
>      .Case("size", VK_SIZE)
> +    .Case("abs8", VK_ABS8)
>      .Case("l", VK_PPC_LO)
>      .Case("h", VK_PPC_HI)
>      .Case("ha", VK_PPC_HA)
> Index: llvm/lib/MC/MCAssembler.cpp
> ===================================================================
> --- llvm/lib/MC/MCAssembler.cpp
> +++ llvm/lib/MC/MCAssembler.cpp
> @@ -741,6 +741,10 @@
>    MCValue Target;
>    uint64_t Value;
>    bool Resolved = evaluateFixup(Layout, Fixup, DF, Target, Value);
> +  if (Target.getSymA() &&
> +      Target.getSymA()->getKind() == MCSymbolRefExpr::VK_ABS8 &&
> +      Fixup.getKind() == FK_Data_1)
> +    return false;
>    return getBackend().fixupNeedsRelaxationAdvanced(Fixup, Resolved, Value, DF,
>                                                     Layout);
>  }
> Index: llvm/include/llvm/MC/MCExpr.h
> ===================================================================
> --- llvm/include/llvm/MC/MCExpr.h
> +++ llvm/include/llvm/MC/MCExpr.h
> @@ -190,6 +190,7 @@
>      VK_SECREL,
>      VK_SIZE,      // symbol at SIZE
>      VK_WEAKREF,   // The link between the symbols in .weakref foo, bar
> +    VK_ABS8,
>  
>      VK_ARM_NONE,
>      VK_ARM_GOT_PREL,
> Index: llvm/docs/Extensions.rst
> ===================================================================
> --- llvm/docs/Extensions.rst
> +++ llvm/docs/Extensions.rst
> @@ -207,6 +207,17 @@
>  Target Specific Behaviour
>  =========================
>  
> +X86
> +---
> +
> +Relocations
> +^^^^^^^^^^^
> +
> +``@ABS8`` can be applied to symbols which appear as immediate operands to
> +instructions that have a 8-bit immediate form for that operand. It causes
> +the assembler to use the 8-bit form and an 8-bit relocation (e.g. ``R_386_8``
> +or ``R_X86_64_8``) for the symbol.
> +
>  Windows on ARM
>  --------------
>  
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