[llvm] [SimplifyCFG] handle monotonic wrapped case for D150943 (PR #65882)

[via llvm-commits]

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@@ -2039,3 +2039,32 @@ return:
   %x = phi i8 [ 3, %sw.default ], [ 124, %sw.bb3 ], [ -99, %sw.bb2 ], [ -66, %sw.bb1 ], [ -33, %entry ]
   ret i8 %x
 }
+
+define i8 @linearmap_dec_wrapped_mon(i3 %0) {                              ; Assume 1
+; CHECK-LABEL: @linearmap_dec_wrapped_mon(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[SWITCH_TABLEIDX:%.*]] = sub i3 [[TMP0:%.*]], -2
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ult i3 [[SWITCH_TABLEIDX]], -4
+; CHECK-NEXT:    [[SWITCH_IDX_MULT:%.*]] = mul i3 [[SWITCH_TABLEIDX]], 2
+; CHECK-NEXT:    [[SWITCH_OFFSET:%.*]] = add i3 [[SWITCH_IDX_MULT]], -4
+; CHECK-NEXT:    [[COND:%.*]] = select i1 [[TMP1]], i3 [[SWITCH_OFFSET]], i3 2
+; CHECK-NEXT:    [[CONV:%.*]] = sext i3 [[COND]] to i8
+; CHECK-NEXT:    ret i8 [[CONV]]
+;
+entry:
+  switch i3 %0, label %cond.end [
+  i3 -1, label %cond.false
+  i3 -2, label %cond.false
+  i3 1, label %cond.false                          ; -> %cond.false
+  i3 0, label %cond.false
+  ]
+
+cond.false:                                       ; preds = %entry, %entry, %entry, %entry
+  %mul = shl nsw i3 %0, 1                            ; 1 << 1 = 2
+  br label %cond.end
+
+cond.end:                                         ; preds = %entry, %cond.false
+  %cond = phi i3 [ %mul, %cond.false ], [ 2, %entry ]; 2
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mikaelholmen wrote:

and here

https://github.com/llvm/llvm-project/pull/65882