[llvm-commits] [llvm] r135103 - /llvm/trunk/lib/Target/X86/X86FrameLowering.cpp

[Chris Lattner]

On Jul 13, 2011, at 4:03 PM, Bill Wendling wrote:
> Author: void
> Date: Wed Jul 13 18:03:31 2011
> New Revision: 135103
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=135103&view=rev
> Log:
> Add code to handle a "frameless" unwind stack.
> 
> The frameless unwind stack has a special encoding, the algorithm for which is in
> "permuteEncode".

Ok, but...

> +++ llvm/trunk/lib/Target/X86/X86FrameLowering.cpp Wed Jul 13 18:03:31 2011
> @@ -1031,13 +1031,55 @@
>   }
> }
> 
> +static uint32_t permuteEncode(unsigned SavedCount, unsigned Registers[6]) {

This function needs a big doxygen comment explaining what it does, and needs comments through its body explaining "how".

-Chris

> +   uint32_t RenumRegs[6];
> +   for (int i = 6 - SavedCount; i < 6; ++i) {
> +       int countless = 0;
> +       for (int j = 6 - SavedCount; j < i; ++j)
> +           if (Registers[j] < Registers[i])
> +               ++countless;
> +
> +       RenumRegs[i] = Registers[i] - countless - 1;
> +   }
> +
> +   uint32_t permutationEncoding = 0;
> +   switch (SavedCount) {
> +   case 6:
> +     permutationEncoding |= 120 * RenumRegs[0] + 24 * RenumRegs[1] 
> +                            + 6 * RenumRegs[2] +  2 * RenumRegs[3] 
> +                            +     RenumRegs[4];
> +     break;
> +   case 5:
> +     permutationEncoding |= 120 * RenumRegs[1] + 24 * RenumRegs[2] 
> +                            + 6 * RenumRegs[3] +  2 * RenumRegs[4] 
> +                            +     RenumRegs[5];
> +     break;
> +   case 4:
> +     permutationEncoding |= 60 * RenumRegs[2] + 12 * RenumRegs[3] 
> +                           + 3 * RenumRegs[4] +      RenumRegs[5];
> +     break;
> +   case 3:
> +     permutationEncoding |= 20 * RenumRegs[3] + 4 * RenumRegs[4] 
> +                               + RenumRegs[5];
> +     break;
> +   case 2:
> +     permutationEncoding |=  5 * RenumRegs[4] + RenumRegs[5];
> +     break;
> +   case 1:
> +     permutationEncoding |= RenumRegs[5];
> +     break;
> +   }
> +   return permutationEncoding;
> +}
> +
> uint32_t X86FrameLowering::
> getCompactUnwindEncoding(ArrayRef<MCCFIInstruction> Instrs,
>                          int DataAlignmentFactor, bool IsEH) const {
>   uint32_t Encoding = 0;
>   int CFAOffset = 0;
>   const TargetRegisterInfo *TRI = TM.getRegisterInfo();
> -  SmallVector<unsigned, 8> SavedRegs;
> +  unsigned SavedRegs[6] = { 0, 0, 0, 0, 0, 0 };
> +  unsigned SavedRegIdx = 0;
>   int FramePointerReg = -1;
> 
>   for (ArrayRef<MCCFIInstruction>::const_iterator
> @@ -1102,9 +1144,10 @@
>       return 0;
>     } else if (Reg < 64) {
>       // DW_CFA_offset + Reg
> +      if (SavedRegIdx >= 6) return 0;
>       int CURegNum = TRI->getCompactUnwindRegNum(Reg, IsEH);
>       if (CURegNum == -1) return 0;
> -      SavedRegs.push_back(CURegNum);
> +      SavedRegs[SavedRegIdx++] = CURegNum;
>     } else {
>       // FIXME: Handle?
>       // DW_CFA_offset_extended
> @@ -1112,6 +1155,9 @@
>     }
>   }
> 
> +  // Bail if there are too many registers to encode.
> +  if (SavedRegIdx > 6) return 0;
> +
>   // Check if the offset is too big.
>   CFAOffset /= 4;
>   if ((CFAOffset & 0xFF) != CFAOffset)
> @@ -1119,20 +1165,16 @@
>   Encoding |= (CFAOffset & 0xFF) << 16; // Size encoding.
> 
>   if (FramePointerReg != -1) {
> -    // Bail if there are too many registers to encode.
> -    if (SavedRegs.size() - 1 > 5) return 0;
> -
> -    Encoding |= 1 << 24;        // EBP/RBP Unwind Frame
> -
> -    unsigned Idx = 0;
> -    for (SmallVectorImpl<unsigned>::iterator
> -           I = SavedRegs.begin(), E = SavedRegs.end(); I != E; ++I) {
> -      unsigned Reg = *I;
> +    Encoding |= 0x01000000;     // EBP/RBP Unwind Frame
> +    for (unsigned I = 0; I != SavedRegIdx; ++I) {
> +      unsigned Reg = SavedRegs[I];
>       if (Reg == unsigned(FramePointerReg)) continue;
> -      Encoding |= (Reg & 0x7) << (Idx++ * 3); // Register encoding
> +      Encoding |= (Reg & 0x7) << (I * 3); // Register encoding
>     }
>   } else {
> -    // FIXME: Handle frameless version!
> +    Encoding |= 0x02000000;     // Frameless unwind with small stack
> +    Encoding |= (SavedRegIdx & 0x7) << 10;
> +    Encoding |= permuteEncode(SavedRegIdx, SavedRegs);
>   }
> 
>   return Encoding;
> 
> 
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