[PATCH] D118663: [AArch64] Adds SUBS and ADDS instructions to the MIPeepholeOpt.

[Dave Green via Phabricator via llvm-commits]

dmgreen added a comment.

Someone once wrote examineCFlagsUse for doing something very similar in the optimizeCompareInstr (because these class as compare instructions). Can some of that be re-used here?

The alternative might be to scan backward through instructions in the main loop, keeping a track of which parts of NZCV are "alive" as we go, so we always have that info available.



================
Comment at: llvm/lib/Target/AArch64/AArch64MIPeepholeOpt.cpp:308
+         "Instruction must only implicit define NZCV");
+  Register NZCV = MI.implicit_operands().begin()->getReg();
+  for (MachineBasicBlock::iterator Iter = std::next(MI.getIterator()),
----------------
Will this always be AArch64::NZCV?


================
Comment at: llvm/lib/Target/AArch64/AArch64MIPeepholeOpt.cpp:343
+    // Instruction defined NZCV so we can stop scanning
+    if (WritesNZCV)
+      break;
----------------
We can stop scanning as soon as the NZCV operand is marked as Dead too, I think.


================
Comment at: llvm/lib/Target/AArch64/AArch64MIPeepholeOpt.cpp:346
+  }
+  return true;
+}
----------------
On rare occasion the NZCV might be live-out of the block (which is probably enough reason to just return false, as opposed to trying to scan further blocks).


Repository:
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  https://reviews.llvm.org/D118663/new/

https://reviews.llvm.org/D118663