OptimizeInstructions: bool(x) xor 1 ==> !bool(x) (#3741)

This was noticed by samparker on LLVM:

https://reviews.llvm.org/D99171

This is apparently a pattern LLVM emits, and doing it there helps by 1-2%
on the real-world Bullet Physics codebase. Seems worthwhile doing here
as well.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index 7e6b00212..be89257a1 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -1698,6 +1698,20 @@ private:
         Bits::getMaxBits(left, this) == 1) {
       return builder.makeUnary(Abstract::getUnary(type, EqZ), left);
     }
+    // bool(x)  ^ 1  ==>  !bool(x)
+    if (matches(curr, binary(Xor, any(&left), ival(1))) &&
+        Bits::getMaxBits(left, this) == 1) {
+      auto* result = builder.makeUnary(Abstract::getUnary(type, EqZ), left);
+      if (left->type == Type::i64) {
+        // Xor's result is also an i64 in this case, but EqZ returns i32, so we
+        // must expand it so that we keep returning the same value as before.
+        // This means we replace a xor and a const with a xor and an extend,
+        // which is still smaller (the const is 2 bytes, the extend just 1), and
+        // also the extend may be removed by further work.
+        result = builder.makeUnary(ExtendUInt32, result);
+      }
+      return result;
+    }
     // bool(x) | 1  ==>  1
     if (matches(curr, binary(Or, pure(&left), ival(1))) &&
         Bits::getMaxBits(left, this) == 1) {