Provide 64bit division in 32bit environments

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

diff --git a/include/arch/x86/div.h b/include/arch/x86/div.h
new file mode 100644 (file)
index 0000000..1985b2d
--- /dev/null
+++ b/include/arch/x86/div.h
@@ -0,0 +1,64 @@
+#ifndef XTF_X86_DIV_H
+#define XTF_X86_DIV_H
+
+#include <stdint.h>
+
+/*
+ * Divide a 64bit number by 32bit divisor without software support.
+ *
+ * The dividend is modified in place, and the modulus is returned.
+ */
+static inline uint32_t divmod64(uint64_t *dividend, uint32_t divisor)
+{
+    uint32_t mod;
+
+#ifdef __x86_64__
+
+    /*
+     * On 64bit, issue a straight 'div' instruction.
+     */
+
+    mod = *dividend %  divisor;
+          *dividend /= divisor;
+#else
+    {
+        /*
+         * On 32bit, this is harder.
+         *
+         * In x86, 'divl' can take a 64bit dividend, but the resulting
+         * quotient must fit in %eax or a #DE will occur.
+         *
+         * To avoid this, we split the division in two.  The remainder from
+         * the higher divide can safely be used in the upper 32bits of the
+         * lower divide, as it will not cause an overflow.
+         */
+        uint32_t high = *dividend >> 32, low = *dividend, umod = 0;
+
+        if ( high )
+        {
+            umod = high %  divisor;
+                   high /= divisor;
+        }
+
+        asm ("divl %2"
+             : "=a" (low), "=d" (mod)
+             : "rm" (divisor), "0" (low), "1" (umod));
+
+        *dividend = (((uint64_t)high) << 32) | low;
+    }
+#endif
+
+    return mod;
+}
+
+#endif /* XTF_X86_DIV_H */
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * tab-width: 4
+ * indent-tabs-mode: nil
+ * End:
+ */