Add x86-specific instruction for i64x2 multiplication

Without this special instruction, legalizing to the AVX512 instruction AND the SSE instruction sequence is impossible. This extra instruction would be rendered unnecessary by the x64 backend.

cranelift/codegen/meta/src/isa/x86/encodings.rs

@@ -1645,6 +1645,7 @@ fn define_simd(
     let x86_pmaxu = x86.by_name("x86_pmaxu");
     let x86_pmins = x86.by_name("x86_pmins");
     let x86_pminu = x86.by_name("x86_pminu");
+    let x86_pmullq = x86.by_name("x86_pmullq");
     let x86_pshufb = x86.by_name("x86_pshufb");
     let x86_pshufd = x86.by_name("x86_pshufd");
     let x86_psll = x86.by_name("x86_psll");
@@ -2101,9 +2102,8 @@ fn define_simd(
 
     // SIMD integer multiplication for I64x2 using a AVX512.
     {
-        let imul = imul.bind(vector(I64, sse_vector_size));
         e.enc_32_64_maybe_isap(
-            imul,
+            x86_pmullq,
             rec_evex_reg_vvvv_rm_128.opcodes(&PMULLQ).w(),
             Some(use_avx512dq_simd), // TODO need an OR predicate to join with AVX512VL
         );

cranelift/codegen/meta/src/isa/x86/instructions.rs

@@ -532,6 +532,23 @@ pub(crate) fn define(
         .operands_out(vec![a]),
     );
 
+    let x = &Operand::new("x", I64x2);
+    let y = &Operand::new("y", I64x2);
+    let a = &Operand::new("a", I64x2);
+    ig.push(
+        Inst::new(
+            "x86_pmullq",
+            r#"
+        Multiply Packed Integers -- Multiply two 64x2 integers and receive a 64x2 result with
+        lane-wise wrapping if the result overflows. This instruction is necessary to add distinct
+        encodings for CPUs with newer vector features.
+        "#,
+            &formats.binary,
+        )
+        .operands_in(vec![x, y])
+        .operands_out(vec![a]),
+    );
+
     let x = &Operand::new("x", TxN);
     let y = &Operand::new("y", TxN);
     let f = &Operand::new("f", iflags);

cranelift/codegen/meta/src/isa/x86/legalize.rs

@@ -359,6 +359,7 @@ fn define_simd(shared: &mut SharedDefinitions, x86_instructions: &InstructionGro
     let icmp = insts.by_name("icmp");
     let imax = insts.by_name("imax");
     let imin = insts.by_name("imin");
+    let imul = insts.by_name("imul");
     let ineg = insts.by_name("ineg");
     let insertlane = insts.by_name("insertlane");
     let ishl = insts.by_name("ishl");
@@ -763,6 +764,12 @@ fn define_simd(shared: &mut SharedDefinitions, x86_instructions: &InstructionGro
         );
     }
 
+    // SIMD imul
+    {
+        let imul = imul.bind(vector(I64, sse_vector_size));
+        narrow.legalize(def!(c = imul(a, b)), vec![def!(c = x86_pmullq(a, b))]);
+    }
+
     narrow.custom_legalize(shuffle, "convert_shuffle");
     narrow.custom_legalize(extractlane, "convert_extractlane");
     narrow.custom_legalize(insertlane, "convert_insertlane");