[LV] Fix PR34523 - avoid generating redundant selects

[Ayal]

Summary

When converting a PHI into a series of 'select' instructions to combine the
incoming values together according their edge masks, initialize the first
value to the incoming value In0 of the first predecessor, instead of
generating a redundant identity 'select(Cond[0], In0, In0)'. The latter
fails when the Cond[0] mask of the first predecessor is null, representing
a full mask, which can happen only when there's a single incoming value.

Added the testcase from the PR.

This fix follows D35725, which introduced using null to represent full masks.

[uabelho]

I don't know this code, but at least the patch solves the crash I reported in PR34523 so thumbs up for that.

[rengolin]

I can't see anything immediately wrong with this patch, it looks trivial and correct. Moreover, if this was done because of a know problem (unlikely, given the commit it came from), we have the assert to help.

LGTM. Thanks!

lib/Transforms/Vectorize/LoopVectorize.cpp

if (P->getParent() != OrigLoop->getHeader()) {
     //                   ( ...)))
     VectorParts Entry(UF);
     for (unsigned In = 0; In < NumIncoming; In++) {
       VectorParts Cond =
           createEdgeMask(P->getIncomingBlock(In), P->getParent());
 
       for (unsigned Part = 0; Part < UF; ++Part) {
         Value *In0 = getOrCreateVectorValue(P->getIncomingValue(In), Part);
-        // We might have single edge PHIs (blocks) - use an identity
-        // 'select' for the first PHI operand.
+        assert((Cond[Part] || NumIncoming == 1) &&
+               "Multiple predecessors with one predecessor having a full mask");
         if (In == 0)
-          Entry[Part] = Builder.CreateSelect(Cond[Part], In0, In0);
+          Entry[Part] = In0; // Initialize with the first incoming value.
         else
           // Select between the current value and the previous incoming edge
           // based on the incoming mask.
           Entry[Part] = Builder.CreateSelect(Cond[Part], In0, Entry[Part],
                                              "predphi");
       }
     }
     for (unsigned Part = 0; Part < UF; ++Part)

test/Transforms/LoopVectorize/if-conversion.ll

cond.end:
   %or = or i32 %or2, %cond
   %inc = add nsw i32 %inc3, 1
   %cmp = icmp slt i32 %inc, 128
   br i1 %cmp, label %for.body, label %for.end
 
 for.end:
   ret i32 %or
 }
+
+; Handle PHI with single incoming value having a full mask.
+; PR34523
+
+; CHECK-LABEL: PR34523
+; CHECK: vector.body
+
+define void @PR34523() {
+bb1:
+  br label %bb2
+
+bb2:                                             ; preds = %bb4, %bb1
+  %i = phi i16 [ undef, %bb1 ], [ %_tmp2, %bb4 ]
+  br label %bb3
+
+bb3:                                             ; preds = %bb2
+  %_tmp1 = phi [1 x [1 x i32]]* [ undef, %bb2 ]
+  br label %bb4
+
+bb4:                                             ; preds = %bb3
+  %_tmp2 = add i16 %i, 1
+  %_tmp3 = icmp slt i16 %_tmp2, 2
+  br i1 %_tmp3, label %bb2, label %bb5
+
+bb5:                                             ; preds = %bb4
+  unreachable
+}