[PATCH] D78439: [AVR] Fix miscompilation of zext + add

[Ayke via Phabricator via llvm-commits]

aykevl added a comment.

Patch committed!

I looked for other instances of this bug, see below.

  def : Pat<(add i16:$src1, imm0_63_neg:$src2),
            (SBIWRdK i16:$src1, (imm0_63_neg:$src2))>;
  def : Pat<(add i16:$src1, imm:$src2),
            (SUBIWRdK i16:$src1, (imm16_neg_XFORM imm:$src2))>;

Both don't involve a carry bit, so should be fine.

  def : Pat<(addc i16:$src1, imm:$src2),
            (SUBIWRdK i16:$src1, (imm16_neg_XFORM imm:$src2))>;

`addc` means "add with carry" while `subiw` is expanded to `subi`/`sbci` which discards the incoming carry bit. This may be a bug.

  def : Pat<(add i8:$src1, imm:$src2),
            (SUBIRdK i8:$src1, (imm8_neg_XFORM imm:$src2))>;

No carry bits, so should be fine.

  def : Pat<(addc i8:$src1, imm:$src2),
            (SUBIRdK i8:$src1, (imm8_neg_XFORM imm:$src2))>;

Like `addc i16`, this discards the carry bit so may be a bug.

  def : Pat<(adde i8:$src1, imm:$src2),
            (SBCIRdK i8:$src1, (imm8_neg_XFORM imm:$src2))>;

`adde` means an add with a carry input and carry output. The `sbci` instruction does the same thing, except that it uses the carry in the wrong direction (subtracting instead of adding). Basically the 8-bit version of the bug this patch is about.

I don't have any code to check, but will leave this here for future reference.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D78439/new/

https://reviews.llvm.org/D78439