[X86][NFC] Compress CD8_Scale from 7 bits to 3 bits

[skan]

Summary

Post: https://discourse.llvm.org/t/save-some-bits-in-tsflags-for-x86/69025

[craig.topper]

llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp

322

I'm not sure this assert as any value. Can just use unsigned?

[skan]

Address review comments

[skan]

llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp

322

Good suggestion! Done.

[craig.topper]

LGTM

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

enum : uint64_t {
     EVEX_L2     = 1ULL << EVEX_L2Shift,
 
     // EVEX_B - Set if this instruction has EVEX.B field set.
     EVEX_BShift = EVEX_L2Shift + 1,
     EVEX_B      = 1ULL << EVEX_BShift,
 
     // The scaling factor for the AVX512's 8-bit compressed displacement.
     CD8_Scale_Shift = EVEX_BShift + 1,
-    CD8_Scale_Mask = 127ULL << CD8_Scale_Shift,
+    CD8_Scale_Mask = 7ULL << CD8_Scale_Shift,
 
     /// Explicitly specified rounding control
-    EVEX_RCShift = CD8_Scale_Shift + 7,
+    EVEX_RCShift = CD8_Scale_Shift + 3,
     EVEX_RC = 1ULL << EVEX_RCShift,
 
     // NOTRACK prefix
     NoTrackShift = EVEX_RCShift + 1,
     NOTRACK = 1ULL << NoTrackShift,
 
     // Force VEX encoding
     ExplicitVEXShift = NoTrackShift + 1,

llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp

 }
 
 /// Determine if this immediate can fit in a disp8 or a compressed disp8 for
 /// EVEX instructions. \p will be set to the value to pass to the ImmOffset
 /// parameter of emitImmediate.
 static bool isDispOrCDisp8(uint64_t TSFlags, int Value, int &ImmOffset) {
   bool HasEVEX = (TSFlags & X86II::EncodingMask) == X86II::EVEX;
 
-  int CD8_Scale =
+  unsigned CD8_Scale =
       (TSFlags & X86II::CD8_Scale_Mask) >> X86II::CD8_Scale_Shift;
-  if (!HasEVEX || CD8_Scale == 0)
+  CD8_Scale = CD8_Scale ? 1U << (CD8_Scale - 1) : 0U;
+  if (!HasEVEX || !CD8_Scale)
     return isInt<8>(Value);
 
   assert(isPowerOf2_32(CD8_Scale) && "Unexpected CD8 scale!");
   if (Value & (CD8_Scale - 1)) // Unaligned offset
     return false;
 
-  int CDisp8 = Value / CD8_Scale;
+  int CDisp8 = Value / static_cast<int>(CD8_Scale);
   if (!isInt<8>(CDisp8))
     return false;
 
   // ImmOffset will be added to Value in emitImmediate leaving just CDisp8.
   ImmOffset = CDisp8 - Value;
   return true;
 }
 

llvm/lib/Target/X86/X86InstrFormats.td

class X86Inst<bits<8> opcod, Format f, ImmType i, dag outs, dag ins,
   let TSFlags{30-29} = OpEncBits;
   let TSFlags{38-31} = Opcode;
   let TSFlags{39}    = hasVEX_4V;
   let TSFlags{40}    = hasVEX_L;
   let TSFlags{41}    = hasEVEX_K;
   let TSFlags{42}    = hasEVEX_Z;
   let TSFlags{43}    = hasEVEX_L2;
   let TSFlags{44}    = hasEVEX_B;
-  // If we run out of TSFlags bits, it's possible to encode this in 3 bits.
-  let TSFlags{51-45} = CD8_Scale;
-  let TSFlags{52}    = hasEVEX_RC;
-  let TSFlags{53}    = hasNoTrackPrefix;
-  let TSFlags{54}    = ExplicitVEXPrefix;
+  let TSFlags{47-45} = !if(!eq(CD8_Scale, 0), 0, !add(!logtwo(CD8_Scale), 1));
+  let TSFlags{48}    = hasEVEX_RC;
+  let TSFlags{49}    = hasNoTrackPrefix;
+  let TSFlags{50}    = ExplicitVEXPrefix;
 }
 
 class PseudoI<dag oops, dag iops, list<dag> pattern>
   : X86Inst<0, Pseudo, NoImm, oops, iops, ""> {
   let Pattern = pattern;
 }
 
 class I<bits<8> o, Format f, dag outs, dag ins, string asm,