LoopVectorize/iv-select-cmp: add test for decreasing IV, const start

[artagnon]

Summary

The most straightforward extension to D150851 would involve a loop with
decreasing induction variable, with a constant start value.
iv-select-cmp.ll only contains a negative test for the decreasing
induction variable case when the start value is variable, namely
not_vectorized_select_decreasing_induction_icmp. Hence, add a test for
the most straightforward extension to D150851, in preparation to
vectorize:

long rdx = 331;
for (long i = 19999; i >= 0; i--) {
  if (a[i] > 3)
    rdx = i;
}
return rdx;

[artagnon]

Remove noundef in argument.

[Mel-Chen]

LG

llvm/test/Transforms/LoopVectorize/iv-select-cmp.ll

218–219

nit:

; CHECK-LABEL: @not_vectorized_select_decreasing_induction_icmp_const_start
; CHECK-NOT:   vector.body
;

llvm/test/Transforms/LoopVectorize/iv-select-cmp.ll

for.body:                                         ; preds = %entry, %for.body
   %inc = add nuw nsw i64 %iv, 1
   %exitcond.not = icmp eq i64 %inc, %n
   br i1 %exitcond.not, label %exit, label %for.body
 
 exit:                                             ; preds = %for.body
   ret float %cond
 }
 
-define i64 @not_vectorized_select_decreasing_induction_icmp(ptr nocapture readonly %a, ptr nocapture readonly %b, i64 %rdx.start, i64 %n) {
-; CHECK-LABEL: @not_vectorized_select_decreasing_induction_icmp
+define i64 @not_vectorized_select_decreasing_induction_icmp_const_start(ptr nocapture readonly %a) {
+; CHECK-LABEL: @not_vectorized_select_decreasing_induction_icmp_const_start
+; CHECK-NOT:   vector.body:
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %entry, %for.body
+  %iv = phi i64 [ 19999, %entry ], [ %dec, %for.body ]
+  %rdx = phi i64 [ 331, %entry ], [ %spec.select, %for.body ]
+  %arrayidx = getelementptr inbounds i64, ptr %a, i64 %iv
+  %0 = load i64, ptr %arrayidx, align 8
+  %cmp = icmp sgt i64 %0, 3
+  %spec.select = select i1 %cmp, i64 %iv, i64 %rdx
+  %dec = add nsw i64 %iv, -1
+  %cmp.not = icmp eq i64 %iv, 0
+  br i1 %cmp.not, label %exit, label %for.body
+
+exit:                                             ; preds = %for.body
+  ret i64 %spec.select
+}
+
+define i64 @not_vectorized_select_decreasing_induction_icmp_non_const_start(ptr nocapture readonly %a, ptr nocapture readonly %b, i64 %rdx.start, i64 %n) {
+; CHECK-LABEL: @not_vectorized_select_decreasing_induction_icmp_non_const_start
 ; CHECK-NOT:   vector.body:
 ;
 entry:
   br label %for.body
 
 for.body:                                         ; preds = %entry, %for.body
   %i.0.in10 = phi i64 [ %iv, %for.body ], [ %n, %entry ]
   %rdx = phi i64 [ %cond, %for.body ], [ %rdx.start, %entry ]