From db207b98a0b32cc881a5ec23169d4c727f352c06 Mon Sep 17 00:00:00 2001
From: Andrew Adams <andrew.b.adams@gmail.com>
Date: Tue, 17 Oct 2023 09:16:33 -0700
Subject: [PATCH] Mutating if branches in isolation can break reachability
 analysis (#7895)

Fixes #7892
---
 src/Simplify_Stmts.cpp             | 16 ++++++++++------
 test/correctness/fuzz_schedule.cpp | 22 ++++++++++++++++++++++
 2 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/src/Simplify_Stmts.cpp b/src/Simplify_Stmts.cpp
index 09b4aed1036d..fd36c6e4fbd2 100644
--- a/src/Simplify_Stmts.cpp
+++ b/src/Simplify_Stmts.cpp
@@ -558,10 +558,10 @@ Stmt Simplify::visit(const Block *op) {
                equal(if_first->condition, if_next->condition) &&
                is_pure(if_first->condition)) {
         // Two ifs with matching conditions.
-        Stmt then_case = mutate(Block::make(if_first->then_case, if_next->then_case));
+        Stmt then_case = Block::make(if_first->then_case, if_next->then_case);
         Stmt else_case;
         if (if_first->else_case.defined() && if_next->else_case.defined()) {
-            else_case = mutate(Block::make(if_first->else_case, if_next->else_case));
+            else_case = Block::make(if_first->else_case, if_next->else_case);
         } else if (if_first->else_case.defined()) {
             // We already simplified the body of the ifs.
             else_case = if_first->else_case;
@@ -572,7 +572,9 @@ Stmt Simplify::visit(const Block *op) {
         if (if_rest.defined()) {
             result = Block::make(result, if_rest);
         }
-        return result;
+        // We must mutate the entire IfThenElse block without first mutating the
+        // branches to compute reachability accurately.
+        return mutate(result);
     } else if (if_first &&
                if_next &&
                !if_next->else_case.defined() &&
@@ -583,13 +585,15 @@ Stmt Simplify::visit(const Block *op) {
         // the first condition.  The second if can be nested
         // inside the first one, because if it's true the
         // first one must also be true.
-        Stmt then_case = mutate(Block::make(if_first->then_case, if_next));
+        Stmt then_case = Block::make(if_first->then_case, if_next);
         Stmt else_case = if_first->else_case;
         Stmt result = IfThenElse::make(if_first->condition, then_case, else_case);
         if (if_rest.defined()) {
             result = Block::make(result, if_rest);
         }
-        return result;
+        // As above, we must mutate the entire IfThenElse block without first
+        // mutating the branches to compute reachability accurately.
+        return mutate(result);
     } else if (if_first &&
                if_next &&
                is_pure(if_first->condition) &&
@@ -608,7 +612,7 @@ Stmt Simplify::visit(const Block *op) {
         if (if_rest.defined()) {
             result = Block::make(result, if_rest);
         }
-        return result;
+        return mutate(result);
     } else if (op->first.same_as(first) &&
                op->rest.same_as(rest)) {
         return op;
diff --git a/test/correctness/fuzz_schedule.cpp b/test/correctness/fuzz_schedule.cpp
index d5a2a664fec5..adc90ac74a0e 100644
--- a/test/correctness/fuzz_schedule.cpp
+++ b/test/correctness/fuzz_schedule.cpp
@@ -96,6 +96,28 @@ int main(int argc, char **argv) {
         check_blur_output(buf, correct);
     }
 
+    // https://github.com/halide/Halide/issues/7892
+    {
+        Func input("input");
+        Func local_sum("local_sum");
+        Func blurry("blurry");
+        Var x("x"), y("y");
+        RVar yryf;
+        input(x, y) = 2 * x + 5 * y;
+        RDom r(-2, 5, -2, 5, "rdom_r");
+        local_sum(x, y) = 0;
+        local_sum(x, y) += input(x + r.x, y + r.y);
+        blurry(x, y) = cast<int32_t>(local_sum(x, y) / 25);
+        Var xo, xi, xoo, xoi, yo, yi;
+        local_sum.vectorize(x)
+            .split(x, xo, xi, 2, TailStrategy::PredicateStores)
+            .split(xo, xoo, xoi, 4, TailStrategy::RoundUp)
+            .unroll(xoi);
+        local_sum.update(0).unscheduled();
+        Pipeline p({blurry});
+        Buffer<int> buf = p.realize({32, 32});
+    }
+
     printf("Success!\n");
 
     return 0;