[llvm] [SimplifyCFG] Fix the compile crash for invalid upper bound value (PR #71351)

[Nikita Popov via llvm-commits]

================
@@ -107,3 +105,103 @@ lor.end:
   %0 = phi i8 [ 15, %sw.bb0 ], [ 6, %sw.bb1 ], [ 7, %sw.bb2 ], [ 0, %default ]
   ret i8 %0
 }
+
+; Negative test: The default branch is unreachable, also it has no result.
+define i1 @switch_lookup_with_small_i1_default_unreachable(i32 %x) {
+; CHECK-LABEL: @switch_lookup_with_small_i1_default_unreachable(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[AND:%.*]] = and i32 [[X:%.*]], 15
+; CHECK-NEXT:    ret i1 false
+;
+entry:
+  %and = and i32 %x, 15
+  switch i32 %and, label %default [
+  i32 4, label %phi.end
+  i32 2, label %phi.end
+  i32 10, label %phi.end
+  i32 9, label %phi.end
+  i32 1, label %sw.bb1.i
+  i32 3, label %sw.bb1.i
+  i32 5, label %sw.bb1.i
+  i32 0, label %sw.bb1.i
+  i32 6, label %sw.bb1.i
+  i32 7, label %sw.bb1.i
+  i32 8, label %sw.bb1.i
+  ]
+
+sw.bb1.i:                                     ; preds = %entry, %entry, %entry, %entry, %entry, %entry, %entry
+  br label %phi.end
+
+default:                                      ; preds = %entry
+  unreachable
+
+phi.end: ; preds = %sw.bb1.i, %entry, %entry, %entry, %entry
+  %retval = phi i1 [ false, %sw.bb1.i ], [ false, %entry ], [ false, %entry ], [ false, %entry ], [ false, %entry ]
+  ret i1 %retval
+}
+
+; Negative test: The result in default reachable, but its value is not const.
+define i1 @switch_lookup_with_small_i1_default_nonconst(i64 %x) {
+; CHECK-LABEL: @switch_lookup_with_small_i1_default_nonconst(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[AND:%.*]] = and i64 [[X:%.*]], 15
+; CHECK-NEXT:    switch i64 [[AND]], label [[DEFAULT:%.*]] [
+; CHECK-NEXT:      i64 10, label [[LOR_END:%.*]]
+; CHECK-NEXT:      i64 1, label [[LOR_END]]
+; CHECK-NEXT:      i64 2, label [[LOR_END]]
+; CHECK-NEXT:    ]
+; CHECK:       default:
+; CHECK-NEXT:    [[CALL:%.*]] = tail call i1 @foo()
+; CHECK-NEXT:    br label [[LOR_END]]
+; CHECK:       lor.end:
+; CHECK-NEXT:    [[TMP0:%.*]] = phi i1 [ true, [[ENTRY:%.*]] ], [ [[CALL]], [[DEFAULT]] ], [ true, [[ENTRY]] ], [ true, [[ENTRY]] ]
+; CHECK-NEXT:    ret i1 [[TMP0]]
+;
+entry:
+  %and = and i64 %x, 15
+  switch i64 %and, label %default [
+  i64 10, label %lor.end
+  i64 1, label %lor.end
+  i64 2, label %lor.end
+  ]
+
+default:                                          ; preds = %entry
+  %call = tail call i1 @foo()
+  br label %lor.end
+
+lor.end:                                          ; preds = %entry, %entry, %entry, %default
+  %0 = phi i1 [ true, %entry ], [ %call, %default ], [ true, %entry ], [ true, %entry ]
+  ret i1 %0
+}
+
+; Negative test: The upper bound index of switch is swapped.
+define void @switch_lookup_with_nonconst_range(i32 %x, i1 %cond) {
+; CHECK-LABEL: @switch_lookup_with_nonconst_range(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_PREHEADER:%.*]]
+; CHECK:       for.preheader:
+; CHECK-NEXT:    br i1 [[COND:%.*]], label [[FOR_PREHEADER]], label [[LOR_END:%.*]]
+; CHECK:       lor.end:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %for.preheader
+
+for.preheader:                                    ; preds = %for.preheader, %entry
+  %add = add nuw i32 %x, 1                        ; the UpperBound is unconfirmed
+  br i1 %cond, label %for.preheader, label %for.end
+
+for.end:                                          ; preds = %for.preheader
+  switch i32 %add, label %default [
+  i32 0, label %lor.end
+  i32 1, label %lor.end
+  i32 5, label %lor.end
+  ]
+
+default:                                          ; preds = %for.end
+  br label %lor.end
+
+lor.end:                                          ; preds = %default, %for.end, %for.end, %for.end
+  %retval.0.i.i = phi i32 [ 0, %default ], [ 0, %for.end ], [ 0, %for.end ], [ 0, %for.end ]
----------------
nikic wrote:

Unless it's necessary to reproduce the issue, it may make sense to pick change these constants so they aren't all the same (otherwise the switch to lookup optimization is meaningless).

https://github.com/llvm/llvm-project/pull/71351