From 3829e8fd475e4b468865cbd21133c5fec34951df Mon Sep 17 00:00:00 2001
From: Leon Shen <leon@yhls.org>
Date: Sat, 25 May 2019 12:10:55 -0400
Subject: [PATCH] fix bug in block renaming for dead code elimination

---
 base/compiler/ssair/ir.jl | 14 +++++++++-----
 test/compiler/ssair.jl    | 16 ++++++++++++++++
 2 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/base/compiler/ssair/ir.jl b/base/compiler/ssair/ir.jl
index dc811b3c79744..0aec26d444476 100644
--- a/base/compiler/ssair/ir.jl
+++ b/base/compiler/ssair/ir.jl
@@ -499,24 +499,28 @@ mutable struct IncrementalCompact
             cur_bb = 1
             for i = 1:length(bb_rename)
                 if i != 1 && length(blocks[i].preds) == 0
-                    bb_rename[i] = 0
+                    bb_rename[i] = -1
                 else
                     bb_rename[i] = cur_bb
                     cur_bb += 1
                 end
             end
             for i = 1:length(bb_rename)
-                bb_rename[i] == 0 && continue
+                bb_rename[i] == -1 && continue
                 preds, succs = blocks[i].preds, blocks[i].succs
                 # Rename preds
-                for j = 1:length(preds); preds[j] = bb_rename[preds[j]]; end
+                for j = 1:length(preds)
+                    if preds[j] != 0
+                        preds[j] = bb_rename[preds[j]]
+                    end
+                end
                 # Dead blocks get removed from the predecessor list
-                filter!(x->x !== 0, preds)
+                filter!(x->x !== -1, preds)
                 # Rename succs
                 for j = 1:length(succs); succs[j] = bb_rename[succs[j]]; end
             end
             let blocks=blocks
-                result_bbs = BasicBlock[blocks[i] for i = 1:length(blocks) if bb_rename[i] != 0]
+                result_bbs = BasicBlock[blocks[i] for i = 1:length(blocks) if bb_rename[i] != -1]
             end
         else
             bb_rename = Vector{Int}()
diff --git a/test/compiler/ssair.jl b/test/compiler/ssair.jl
index 7603e7b8a0574..30f53b124ed66 100644
--- a/test/compiler/ssair.jl
+++ b/test/compiler/ssair.jl
@@ -97,6 +97,22 @@ let
     end
 end
 
+# PR #32145
+# Make sure IncrementalCompact can handle blocks with predecessors of index 0
+# while removing blocks with no predecessors.
+let cfg = CFG(BasicBlock[
+    make_bb([]        , [2, 4]),
+    make_bb([1]       , [4, 5]),
+    make_bb([]        , [4]   ), # should be removed
+    make_bb([0, 1, 2] , [5]   ), # 0 predecessor should be preserved
+    make_bb([2, 3]    , []    ),
+], Int[])
+    code = Compiler.IRCode(
+        [], [], Int32[], UInt8[], cfg, LineInfoNode[], [], [], [])
+    compact = Compiler.IncrementalCompact(code, true)
+    @test length(compact.result_bbs) == 4 && 0 in compact.result_bbs[3].preds
+end
+
 # Issue #32579 - Optimizer bug involving type constraints
 function f32579(x::Int, b::Bool)
     if b