MC: Introduce the ABS8 symbol modifier.

@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. Differential Revision: https://reviews.llvm.org/D28688 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293667 91177308-0d34-0410-b5e6-96231b3b80d8
zmyer · Jan 31, 2017 · 043d1a9 · 043d1a9
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diff --git a/docs/Extensions.rst b/docs/Extensions.rst
@@ -207,6 +207,28 @@ in the assembler to differentiate the sections.
 Target Specific Behaviour
 =========================
 
+X86
+---
+
+Relocations
+^^^^^^^^^^^
+
+``@ABS8`` can be applied to symbols which appear as immediate operands to
+instructions that have an 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.
+
+For example:
+
+.. code-block:: gas
+
+  cmpq $foo@ABS8, %rdi
+
+This causes the assembler to select the form of the 64-bit ``cmpq`` instruction
+that takes an 8-bit immediate operand that is sign extended to 64 bits, as
+opposed to ``cmpq $foo, %rdi`` which takes a 32-bit immediate operand. This
+is also not the same as ``cmpb $foo, %dil``, which is an 8-bit comparison.
+
 Windows on ARM
 --------------
 

diff --git a/include/llvm/MC/MCExpr.h b/include/llvm/MC/MCExpr.h
@@ -194,6 +194,8 @@ class MCSymbolRefExpr : public MCExpr {
     VK_SIZE,      // symbol@SIZE
     VK_WEAKREF,   // The link between the symbols in .weakref foo, bar
 
+    VK_X86_ABS8,
+
     VK_ARM_NONE,
     VK_ARM_GOT_PREL,
     VK_ARM_TARGET1,

diff --git a/lib/MC/MCAssembler.cpp b/lib/MC/MCAssembler.cpp
@@ -747,6 +747,10 @@ bool MCAssembler::fixupNeedsRelaxation(const MCFixup &Fixup,
   MCValue Target;
   uint64_t Value;
   bool Resolved = evaluateFixup(Layout, Fixup, DF, Target, Value);
+  if (Target.getSymA() &&
+      Target.getSymA()->getKind() == MCSymbolRefExpr::VK_X86_ABS8 &&
+      Fixup.getKind() == FK_Data_1)
+    return false;
   return getBackend().fixupNeedsRelaxationAdvanced(Fixup, Resolved, Value, DF,
                                                    Layout);
 }

diff --git a/lib/MC/MCExpr.cpp b/lib/MC/MCExpr.cpp
@@ -208,6 +208,7 @@ StringRef MCSymbolRefExpr::getVariantKindName(VariantKind Kind) {
   case VK_SECREL: return "SECREL32";
   case VK_SIZE: return "SIZE";
   case VK_WEAKREF: return "WEAKREF";
+  case VK_X86_ABS8: return "ABS8";
   case VK_ARM_NONE: return "none";
   case VK_ARM_GOT_PREL: return "GOT_PREL";
   case VK_ARM_TARGET1: return "target1";
@@ -317,6 +318,7 @@ MCSymbolRefExpr::getVariantKindForName(StringRef Name) {
     .Case("imgrel", VK_COFF_IMGREL32)
     .Case("secrel32", VK_SECREL)
     .Case("size", VK_SIZE)
+    .Case("abs8", VK_X86_ABS8)
     .Case("l", VK_PPC_LO)
     .Case("h", VK_PPC_HI)
     .Case("ha", VK_PPC_HA)

diff --git a/lib/Target/X86/MCTargetDesc/X86ELFObjectWriter.cpp b/lib/Target/X86/MCTargetDesc/X86ELFObjectWriter.cpp
@@ -96,6 +96,7 @@ static unsigned getRelocType64(MCContext &Ctx, SMLoc Loc,
   default:
     llvm_unreachable("Unimplemented");
   case MCSymbolRefExpr::VK_None:
+  case MCSymbolRefExpr::VK_X86_ABS8:
     switch (Type) {
     case RT64_64:
       return IsPCRel ? ELF::R_X86_64_PC64 : ELF::R_X86_64_64;
@@ -219,6 +220,7 @@ static unsigned getRelocType32(MCContext &Ctx,
   default:
     llvm_unreachable("Unimplemented");
   case MCSymbolRefExpr::VK_None:
+  case MCSymbolRefExpr::VK_X86_ABS8:
     switch (Type) {
     case RT32_32:
       return IsPCRel ? ELF::R_386_PC32 : ELF::R_386_32;

diff --git a/test/MC/X86/abs8.s b/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@ABS8, %edi