[all-commits] [llvm/llvm-project] bc15bf: [X86] matchAdd: don't fold a large offset into a %...

[Fangrui Song via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: bc15bf66dcca76cc06fe71fca35b74dc4d521021
      https://github.com/llvm/llvm-project/commit/bc15bf66dcca76cc06fe71fca35b74dc4d521021
  Author: Fangrui Song <maskray at google.com>
  Date:   2020-01-28 (Tue, 28 Jan 2020)

  Changed paths:
    M llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
    A llvm/test/CodeGen/X86/fold-add-pcrel.ll

  Log Message:
  -----------
  [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)`,

```
X86DAGToDAGISel::matchAdd
  ...
// 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