[Backend] Fix alias analysis with ub.poison (#7463)

Optimistically assume poison values alias nothing.

Author: Mogball

lib/Analysis/Alias.cpp

@@ -1,5 +1,6 @@
 #include "triton/Analysis/Alias.h"
 
+#include "mlir/Dialect/UB/IR/UBOps.h"
 #include "mlir/Support/LLVM.h"
 #include "triton/Dialect/TritonGPU/IR/Dialect.h"
 
@@ -38,6 +39,9 @@ LogicalResult SharedMemoryAliasAnalysis::visitOperation(
   } else if (op->hasTrait<OpTrait::MemDescViewTrait>()) {
     aliasInfo = AliasInfo(operands[0]->getValue());
     pessimistic = false;
+  } else if (isa<ub::PoisonOp>(op)) {
+    aliasInfo = AliasInfo();
+    pessimistic = false;
   } else {
     assert(!isa<triton::gpu::MemDescType>(result.getType()) &&
            "unknown operation creating memory descriptor");

test/Analysis/test-alias.mlir

@@ -191,4 +191,18 @@ tt.func @cf_for(%arg0: index, %arg1: index, %arg2: index, %arg3: !tt.ptr<f16>, %
   tt.return
 }
 
+tt.func @poison_memdesc(%arg0: i1) {
+  // expected-remark @below {{%0 -> %0}}
+  %0 = ttg.local_alloc : () -> !ttg.memdesc<128x32xf16, #A_SHARED, #ttg.shared_memory, mutable>
+  cf.cond_br %arg0, ^bb1, ^bb2(%0 : !ttg.memdesc<128x32xf16, #A_SHARED, #ttg.shared_memory, mutable>)
+^bb1:
+  %1 = ub.poison : !ttg.memdesc<128x32xf16, #A_SHARED, #ttg.shared_memory, mutable>
+  cf.br ^bb2(%1 : !ttg.memdesc<128x32xf16, #A_SHARED, #ttg.shared_memory, mutable>)
+^bb2(%2: !ttg.memdesc<128x32xf16, #A_SHARED, #ttg.shared_memory, mutable>):
+  %c0_i32 = arith.constant 0 : i32
+  // expected-remark @below {{%3 -> %0}}
+  %3 = ttg.memdesc_subview %2[%c0_i32, %c0_i32] : !ttg.memdesc<128x32xf16, #A_SHARED, #ttg.shared_memory, mutable> -> !ttg.memdesc<128x32xf16, #A_SHARED, #ttg.shared_memory, mutable>
+  tt.return
+}
+
 }  // module