From f070e8c10a15a02735fbd9b88c4c569a8c786933 Mon Sep 17 00:00:00 2001
From: Thomas Lively <7121787+tlively@users.noreply.github.com>
Date: Mon, 26 Aug 2019 12:40:38 -0700
Subject: Do not hoist truncation of wasm2js divisions (#2305)

It is not valid to defer the truncation of divisions because
accumulated non-integral results can produce different values when
they are combined before truncation. This was causing a test failure
in the Rust test suite.
---
 src/tools/wasm2js.cpp | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

(limited to 'src')

diff --git a/src/tools/wasm2js.cpp b/src/tools/wasm2js.cpp
index 2007a65d8..ebed16226 100644
--- a/src/tools/wasm2js.cpp
+++ b/src/tools/wasm2js.cpp
@@ -353,11 +353,13 @@ static void optimizeJS(Ref ast) {
     } else if (isUnary(node, L_NOT)) {
       node[2] = optimizeBoolean(node[2]);
     }
-    // Add/subtract can merge coercions up.
+    // Add/subtract can merge coercions up, except when a child is a division,
+    // which needs to be eagerly truncated to remove fractional results.
     else if (isBinary(node, PLUS) || isBinary(node, MINUS)) {
       auto left = node[2];
       auto right = node[3];
-      if (isOrZero(left) && isOrZero(right)) {
+      if (isOrZero(left) && isOrZero(right) && !isBinary(left[2], DIV) &&
+          !isBinary(right[2], DIV)) {
         auto op = node[1]->getIString();
         // Add a coercion on top.
         node[1]->setString(OR);
-- 
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