X86: Produce @ABS8 symbol modifiers for absolute symbols in range [0,256).

[pcc]

Summary

Depends on D28688

[pcc]

Ping.

[RKSimon]

llvm/lib/Target/X86/X86Subtarget.cpp

100 ↗

(On Diff #84348)

Should we treating this as an unsigned value? D28688 suggested its sign-extended

[pcc]

• Address review comments

[pcc]

llvm/lib/Target/X86/X86Subtarget.cpp

100 ↗

(On Diff #84348)

Good point. The tricky thing here is that R_X86_64_8 is an unsigned relocation, but we're using it to relocate a sign extended operand. Probably the safest thing we can do here is to accept only the range [0,128), which I've done. If it proves necessary later, we can try to recognise patterns like sext i64 (trunc i8 (i8* @foo)) and allow the range [-128,128).

[pcc]

• Update a comment

[RKSimon]

Please update the patch title to "X86: Produce @ABS8 symbol modifiers for absolute symbols in range [0,128)." before submission.

LGTM

llvm/trunk/lib/Target/X86/MCTargetDesc/X86BaseInfo.h

enum TOF {
     ///
     /// This is the 32-bit TLS offset for Darwin TLS in PIC mode.
     MO_TLVP_PIC_BASE,
 
     /// MO_SECREL - On a symbol operand this indicates that the immediate is
     /// 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 {
     //===------------------------------------------------------------------===//
     // Instruction encodings.  These are the standard/most common forms for X86
     // instructions.
     //
 

llvm/trunk/lib/Target/X86/X86MCInstLower.cpp

MCOperand X86MCInstLower::LowerSymbolOperand(const MachineOperand &MO,
   case X86II::MO_TPOFF:     RefKind = MCSymbolRefExpr::VK_TPOFF; break;
   case X86II::MO_DTPOFF:    RefKind = MCSymbolRefExpr::VK_DTPOFF; break;
   case X86II::MO_NTPOFF:    RefKind = MCSymbolRefExpr::VK_NTPOFF; break;
   case X86II::MO_GOTNTPOFF: RefKind = MCSymbolRefExpr::VK_GOTNTPOFF; break;
   case X86II::MO_GOTPCREL:  RefKind = MCSymbolRefExpr::VK_GOTPCREL; break;
   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);
     // Subtract the pic base.
     Expr = MCBinaryExpr::createSub(Expr,
                             MCSymbolRefExpr::create(MF.getPICBaseSymbol(), Ctx),
                                    Ctx);
     if (MO.isJTI()) {

llvm/trunk/lib/Target/X86/X86Subtarget.cpp

 // This file implements the X86 specific subclass of TargetSubtargetInfo.
 //
 //===----------------------------------------------------------------------===//
 
 #include "X86Subtarget.h"
 #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"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/Host.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Target/TargetMachine.h"

 
 unsigned char X86Subtarget::classifyGlobalReference(const GlobalValue *GV,
                                                     const Module &M) const {
   // Large model never uses stubs.
   if (TM.getCodeModel() == CodeModel::Large)
     return X86II::MO_NO_FLAG;
 
   // Absolute symbols can be referenced directly.
-  if (GV && GV->isAbsoluteSymbolRef())
+  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);
 
   if (isTargetCOFF())
     return X86II::MO_DLLIMPORT;
 
   if (is64Bit())

llvm/trunk/test/CodeGen/X86/absolute-rotate.ll

 ; RUN: llc < %s | FileCheck %s
 ; RUN: llc -relocation-model=pic < %s | FileCheck %s
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 @align = external hidden global i8, !absolute_symbol !0
 
 declare void @f()
 
 define void @foo(i64 %val) {
   %shr = lshr i64 %val, zext (i8 ptrtoint (i8* @align to i8) to i64)
   %shl = shl i64 %val, zext (i8 sub (i8 64, i8 ptrtoint (i8* @align to i8)) to i64)
-  ; CHECK: rorq $align, %rdi
+  ; CHECK: rorq $align@ABS8, %rdi
   %ror = or i64 %shr, %shl
   %cmp = icmp ult i64 %ror, 109
   br i1 %cmp, label %t, label %f
 
 t:
   call void @f()
   ret void
 
 f:
   ret void
 }
 
-!0 = !{i64 0, i64 256}
+!0 = !{i64 0, i64 128}