target/riscv: rvv: Source vector registers cannot overlap mask register

Add the relevant ISA paragraphs explaining why source (and destination)
registers cannot overlap the mask register.

Signed-off-by: Anton Blanchard <[email protected]>
Reviewed-by: Daniel Henrique Barboza <[email protected]>
Reviewed-by: Max Chou <[email protected]>
Signed-off-by: Max Chou <[email protected]>
Message-ID: <[email protected]>
Signed-off-by: Alistair Francis <[email protected]>
Cc: [email protected]
(cherry picked from commit 3e8d1e4)
Signed-off-by: Michael Tokarev <[email protected]>

Author: antonblanchard

target/riscv/insn_trans/trans_rvv.c.inc

@@ -100,10 +100,33 @@ static bool require_scale_rvfmin(DisasContext *s)
     }
 }
 
-/* Destination vector register group cannot overlap source mask register. */
-static bool require_vm(int vm, int vd)
+/*
+ * Source and destination vector register groups cannot overlap source mask
+ * register:
+ *
+ * A vector register cannot be used to provide source operands with more than
+ * one EEW for a single instruction. A mask register source is considered to
+ * have EEW=1 for this constraint. An encoding that would result in the same
+ * vector register being read with two or more different EEWs, including when
+ * the vector register appears at different positions within two or more vector
+ * register groups, is reserved.
+ * (Section 5.2)
+ *
+ * A destination vector register group can overlap a source vector
+ * register group only if one of the following holds:
+ *  1. The destination EEW equals the source EEW.
+ *  2. The destination EEW is smaller than the source EEW and the overlap
+ *     is in the lowest-numbered part of the source register group.
+ *  3. The destination EEW is greater than the source EEW, the source EMUL
+ *     is at least 1, and the overlap is in the highest-numbered part of
+ *     the destination register group.
+ * For the purpose of determining register group overlap constraints, mask
+ * elements have EEW=1.
+ * (Section 5.2)
+ */
+static bool require_vm(int vm, int v)
 {
-    return (vm != 0 || vd != 0);
+    return (vm != 0 || v != 0);
 }
 
 static bool require_nf(int vd, int nf, int lmul)