[Mlir-commits] [mlir] [MLIR] Erase unreachable blocks before applying patterns in the greedy rewriter (PR #153957)

[Mehdi Amini]

================
@@ -3363,3 +3363,18 @@ func.func @bf16_fma(%arg0: vector<32x32x32xbf16>, %arg1: vector<32x32x32xbf16>,
     }
   }
 #-}
+
+// CHECK-LABEL: func @unreachable()
+// CHECK-NEXT: return
+// CHECK-NOT: arith
+func.func @unreachable() {
+  return
+^unreachable:
+  %c1_i64 = arith.constant 1 : i64
+  // This self referencing operation is legal in an unreachable block.
+  // Many patterns are unsafe with respect to this kind of situation,
+  // check that we don't infinite loop here.
+  %add = arith.addi %add, %c1_i64 : i64
----------------
joker-eph wrote:

It is because an unreachable block is considering to self-dominate itself: so there is no violation of the dominance.
This isn't an MLIR thing by the way, that's how LLVM works already.

One of the reason why this is really useful is that it preserves the ability to perform local transformation without having to look for "effects at a distance".
That is: simple patterns of replacement doing RAUW, or updating the CFG can easily lead to this situation.


https://github.com/llvm/llvm-project/pull/153957