X86: Produce @ABS8 symbol modifiers for absolute symbols in range [0,128).

Differential Revision: https://reviews.llvm.org/D28689

llvm-svn: 293844

3 files changed, 19 insertions, 3 deletions

diff --git a/llvm/lib/Target/X86/MCTargetDesc/X86BaseInfo.h b/llvm/lib/Target/X86/MCTargetDesc/X86BaseInfo.h
index aab552547fa..d8953da4abb 100644
--- a/llvm/lib/Target/X86/MCTargetDesc/X86BaseInfo.h
+++ b/llvm/lib/Target/X86/MCTargetDesc/X86BaseInfo.h
@@ -212,7 +212,12 @@ namespace X86II {
     /// the offset from beginning of section.
     ///
     /// This is the TLS offset for the COFF/Windows TLS mechanism.
-    MO_SECREL
+    MO_SECREL,
+
+    /// MO_ABS8 - On a symbol operand this indicates that the symbol is known
+    /// to be an absolute symbol in range [0,128), so we can use the @ABS8
+    /// symbol modifier.
+    MO_ABS8,
   };
 
   enum : uint64_t {
diff --git a/llvm/lib/Target/X86/X86MCInstLower.cpp b/llvm/lib/Target/X86/X86MCInstLower.cpp
index c7cc1a08237..fd4626c494e 100644
--- a/llvm/lib/Target/X86/X86MCInstLower.cpp
+++ b/llvm/lib/Target/X86/X86MCInstLower.cpp
@@ -215,6 +215,7 @@ MCOperand X86MCInstLower::LowerSymbolOperand(const MachineOperand &MO,
   case X86II::MO_GOT:       RefKind = MCSymbolRefExpr::VK_GOT; break;
   case X86II::MO_GOTOFF:    RefKind = MCSymbolRefExpr::VK_GOTOFF; break;
   case X86II::MO_PLT:       RefKind = MCSymbolRefExpr::VK_PLT; break;
+  case X86II::MO_ABS8:      RefKind = MCSymbolRefExpr::VK_X86_ABS8; break;
   case X86II::MO_PIC_BASE_OFFSET:
   case X86II::MO_DARWIN_NONLAZY_PIC_BASE:
     Expr = MCSymbolRefExpr::create(Sym, Ctx);
diff --git a/llvm/lib/Target/X86/X86Subtarget.cpp b/llvm/lib/Target/X86/X86Subtarget.cpp
index 586bb7bd7b1..69ed9256248 100644
--- a/llvm/lib/Target/X86/X86Subtarget.cpp
+++ b/llvm/lib/Target/X86/X86Subtarget.cpp
@@ -15,6 +15,7 @@
 #include "X86InstrInfo.h"
 #include "X86TargetMachine.h"
 #include "llvm/IR/Attributes.h"
+#include "llvm/IR/ConstantRange.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/GlobalValue.h"
 #include "llvm/Support/CommandLine.h"
@@ -93,8 +94,17 @@ unsigned char X86Subtarget::classifyGlobalReference(const GlobalValue *GV,
     return X86II::MO_NO_FLAG;
 
   // Absolute symbols can be referenced directly.
-  if (GV && GV->isAbsoluteSymbolRef())
-    return X86II::MO_NO_FLAG;
+  if (GV) {
+    if (Optional<ConstantRange> CR = GV->getAbsoluteSymbolRange()) {
+      // See if we can use the 8-bit immediate form. Note that some instructions
+      // will sign extend the immediate operand, so to be conservative we only
+      // accept the range [0,128).
+      if (CR->getUnsignedMax().ult(128))
+        return X86II::MO_ABS8;
+      else
+        return X86II::MO_NO_FLAG;
+    }
+  }
 
   if (TM.shouldAssumeDSOLocal(M, GV))
     return classifyLocalReference(GV);