[X86] matchAdd: don't fold a large offset into a %rip relative address

Authored by MaskRay on Jan 28 2020, 9:18 PM.


[X86] matchAdd: don't fold a large offset into a %rip relative address

For ret i64 add (i64 ptrtoint (i32* @foo to i64), i64 1701208431),

// AM.setBaseReg(CurDAG->getRegister(X86::RIP, MVT::i64));
  if (!matchAddressRecursively(N.getOperand(0), AM, Depth+1) &&
// Try folding offset but fail; there is a symbolic displacement, so offset cannot be too large
      !matchAddressRecursively(Handle.getValue().getOperand(1), AM, Depth+1))
    return false;
  // Try again after commuting the operands.
// AM.Disp = Val; foldOffsetIntoAddress() does not know there will be a symbolic displacement
  if (!matchAddressRecursively(Handle.getValue().getOperand(1), AM, Depth+1) &&
// AM.setBaseReg(CurDAG->getRegister(X86::RIP, MVT::i64));
      !matchAddressRecursively(Handle.getValue().getOperand(0), AM, Depth+1))
// Succeeded! Produced leaq sym+disp(%rip),...
    return false;

foldOffsetIntoAddress() currently does not know there is a symbolic
displacement and can fold a large offset.

The produced leaq sym+disp(%rip), %rax instruction is relocated by
an R_X86_64_PC32. If disp is large and sym+disp-rip>=2**31, there
will be a relocation overflow.

This approach is still not elegant. Unfortunately the isRIPRelative
interface is a bit clumsy. I tried several solutions and eventually
picked this one.

Differential Revision: https://reviews.llvm.org/D73606