[llvm] [AArch64] Improve expansion of immediates of the form (~w << 32 | w). (PR #162286)

[Ricardo Jesus via llvm-commits]

================
@@ -239,6 +239,59 @@ static bool trySequenceOfOnes(uint64_t UImm,
   return true;
 }
 
+// Attempt to expand 64-bit immediate values whose negated upper half match
+// the lower half (for example, 0x1234'5678'edcb'a987).
+// Immediates of this form can generally be expanded via a sequence of
+// MOVN+MOVK to expand the lower half, followed by an EOR to shift and negate
+// the result to the upper half, e.g.:
+//   mov  x0, #-22137          // =0xffffffffffffa987
+//   movk x0, #60875, lsl #16  // =0xffffffffedcba987
+//   eor  x0, x0, x0, lsl #32  // =0xffffffffedcba987 ^ 0xedcba98700000000
+//                                =0x12345678edcba987.
+// When the lower half contains a 16-bit chunk of ones, such as
+// 0x0000'5678'ffff'a987, the intermediate MOVK is redundant.
+// Similarly, when it contains a 16-bit chunk of zeros, such as
+// 0xffff'5678'0000'a987, the expansion can instead be effected by expanding
+// the negation of the lower half and negating the result with an EON, e.g.:
+//   mov  x0, #-43400          // =0xffffffffffff5678
+//   eon  x0, x0, x0, lsl #32  // =0xffffffffffff5678 ^ ~0xffff567800000000
----------------
rj-jesus wrote:

Apologies for the delay, I didn't manage to get back to this last week. I've extended the code to support shift amounts other than 32. Given the new code structure, it might also be worth adding support for patterns with BIC (they overlap somewhat with EOR, but some instances are unique). If you'd like I'm happy to do so in this PR, otherwise we can revisit this once the PR is merged.

Please let me know what you think. :)

https://github.com/llvm/llvm-project/pull/162286