cbe: fix ub triggered by mulw overflowing the promoted type

Closes #21914

1 files changed, 56 insertions, 0 deletions

diff --git a/test/behavior/math.zig b/test/behavior/math.zig
index fb325a95b6..a658a0559c 100644
--- a/test/behavior/math.zig
+++ b/test/behavior/math.zig
@@ -778,6 +778,62 @@ fn should_not_be_zero(x: f128) !void {
     try expect(x != 0.0);
 }
 
+test "umax wrapped squaring" {
+    if (builtin.zig_backend == .stage2_riscv64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
+
+    {
+        var x: u4 = maxInt(u4);
+        x *%= x;
+        try expect(x == 1);
+    }
+    {
+        var x: u8 = maxInt(u8);
+        x *%= x;
+        try expect(x == 1);
+    }
+    {
+        var x: u12 = maxInt(u12);
+        x *%= x;
+        try expect(x == 1);
+    }
+    {
+        var x: u16 = maxInt(u16);
+        x *%= x;
+        try expect(x == 1);
+    }
+    {
+        var x: u24 = maxInt(u24);
+        x *%= x;
+        try expect(x == 1);
+    }
+    {
+        var x: u32 = maxInt(u32);
+        x *%= x;
+        try expect(x == 1);
+    }
+    {
+        var x: u48 = maxInt(u48);
+        x *%= x;
+        try expect(x == 1);
+    }
+    {
+        var x: u64 = maxInt(u64);
+        x *%= x;
+        try expect(x == 1);
+    }
+    {
+        var x: u96 = maxInt(u96);
+        x *%= x;
+        try expect(x == 1);
+    }
+    {
+        var x: u128 = maxInt(u128);
+        x *%= x;
+        try expect(x == 1);
+    }
+}
+
 test "128-bit multiplication" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO