cfg_simplify: Avoid ambiguous magic number (#52111)

This code was using the sentinel value -1 as a special marker in
addition to the negative BB indices. That ambiguity was causing
`cfg_simplify!` to fail on functions that merge any BB into the first
basic block.

This change is to use `typemin(Int)` as a marker instead.

Fixes #52058

Author: topolarity

base/compiler/ssair/passes.jl

@@ -2029,7 +2029,7 @@ function cfg_simplify!(ir::IRCode)
         i = popfirst!(worklist)
         # Drop blocks that will be merged away
         if merge_into[i] != 0
-            bb_rename_succ[i] = -1
+            bb_rename_succ[i] = typemin(Int)
         end
         # Mark dropped blocks for fixup
         if !isempty(searchsorted(dropped_bbs, i))
@@ -2049,7 +2049,7 @@ function cfg_simplify!(ir::IRCode)
                 # we have to schedule that block next
                 while merged_succ[curr] != 0
                     if bb_rename_succ[curr] == 0
-                        bb_rename_succ[curr] = -1
+                        bb_rename_succ[curr] = typemin(Int)
                     end
                     curr = merged_succ[curr]
                 end
@@ -2089,9 +2089,9 @@ function cfg_simplify!(ir::IRCode)
         resolved_all = true
         for bb in dropped_bbs
             obb = bb_rename_succ[bb]
-            if obb < -1
+            if obb < 0 && obb != typemin(Int)
                 nsucc = bb_rename_succ[-obb]
-                if nsucc == -1
+                if nsucc == typemin(Int)
                     nsucc = -merge_into[-obb]
                 end
                 bb_rename_succ[bb] = nsucc
@@ -2106,6 +2106,8 @@ function cfg_simplify!(ir::IRCode)
         if bb_rename_succ[i] == 0
             bb_rename_succ[i] = -1
             bb_rename_pred[i] = -2
+        elseif bb_rename_succ[i] == typemin(Int)
+            bb_rename_succ[i] = -1
         end
     end
 

test/compiler/irpasses.jl

@@ -1569,4 +1569,19 @@ let m = Meta.@lower 1 + 1
     Core.Compiler.verify_ir(ir)
 end
 
+function f_with_merge_to_entry_block()
+    while true
+        i = @noinline rand(Int)
+        if @noinline isodd(i)
+            return i
+        end
+    end
+end
+
+let (ir, _) = only(Base.code_ircode(f_with_merge_to_entry_block))
+    Core.Compiler.verify_ir(ir)
+    ir = Core.Compiler.cfg_simplify!(ir)
+    Core.Compiler.verify_ir(ir)
+end
+
 # JET.test_opt(Core.Compiler.cfg_simplify!, (Core.Compiler.IRCode,))