Add mulmod

kiwiyou · Dec 9, 2023 · 5da31db · 5da31db
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diff --git a/src/SUMMARY.md b/src/SUMMARY.md
@@ -19,6 +19,7 @@
 - [Linear Recurrence (Constant Coefficient)](rec.md)
 - [Constant Division / Modulo](cdiv.md)
 - [Number Theoretic Transform](ntt.md)
+- [Mulmod](mulmod.md)
 
 # Geometry
 

diff --git a/src/mulmod.md b/src/mulmod.md
@@ -0,0 +1,21 @@
+# Mulmod
+
+Computes 64bit a times b mod c, returning quotient and remainder
+
+```rs
+fn mulmod(a: u64, b: u64, c: u64) -> (u64, u64) {
+    let (quot, rem);
+    unsafe {
+        std::arch::asm!(
+            "mul {b}",
+            "div {c}",
+            inout("rax") a => quot,
+            b = in(reg) b,
+            c = in(reg) c,
+            out("rdx") rem, 
+            options(nomem, pure)
+        )
+    };
+    (quot, rem)
+}
+```