target/arm: Add TB flag for "MVE insns not predicated"

Our current codegen for MVE always calls out to helper functions,
because some byte lanes might be predicated.  The common case is that
in fact there is no predication active and all lanes should be
updated together, so we can produce better code by detecting that and
using the TCG generic vector infrastructure.

Add a TB flag that is set when we can guarantee that there is no
active MVE predication, and a bool in the DisasContext.  Subsequent
patches will use this flag to generate improved code for some
instructions.

In most cases when the predication state changes we simply end the TB
after that instruction.  For the code called from vfp_access_check()
that handles lazy state preservation and creating a new FP context,
we can usually avoid having to try to end the TB because luckily the
new value of the flag following the register changes in those
sequences doesn't depend on any runtime decisions.  We do have to end
the TB if the guest has enabled lazy FP state preservation but not
automatic state preservation, but this is an odd corner case that is
not going to be common in real-world code.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210913095440.13462-4-peter.maydell@linaro.org

1 files changed, 33 insertions, 0 deletions

diff --git a/target/arm/helper.c b/target/arm/helper.c
index 21ee94ec2e..6274221447 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -13637,6 +13637,35 @@ static inline void assert_hflags_rebuild_correctly(CPUARMState *env)
 #endif
 }
 
+static bool mve_no_pred(CPUARMState *env)
+{
+    /*
+     * Return true if there is definitely no predication of MVE
+     * instructions by VPR or LTPSIZE. (Returning false even if there
+     * isn't any predication is OK; generated code will just be
+     * a little worse.)
+     * If the CPU does not implement MVE then this TB flag is always 0.
+     *
+     * NOTE: if you change this logic, the "recalculate s->mve_no_pred"
+     * logic in gen_update_fp_context() needs to be updated to match.
+     *
+     * We do not include the effect of the ECI bits here -- they are
+     * tracked in other TB flags. This simplifies the logic for
+     * "when did we emit code that changes the MVE_NO_PRED TB flag
+     * and thus need to end the TB?".
+     */
+    if (cpu_isar_feature(aa32_mve, env_archcpu(env))) {
+        return false;
+    }
+    if (env->v7m.vpr) {
+        return false;
+    }
+    if (env->v7m.ltpsize < 4) {
+        return false;
+    }
+    return true;
+}
+
 void cpu_get_tb_cpu_state(CPUARMState *env, target_ulong *pc,
                           target_ulong *cs_base, uint32_t *pflags)
 {
@@ -13676,6 +13705,10 @@ void cpu_get_tb_cpu_state(CPUARMState *env, target_ulong *pc,
             if (env->v7m.fpccr[is_secure] & R_V7M_FPCCR_LSPACT_MASK) {
                 DP_TBFLAG_M32(flags, LSPACT, 1);
             }
+
+            if (mve_no_pred(env)) {
+                DP_TBFLAG_M32(flags, MVE_NO_PRED, 1);
+            }
         } else {
             /*
              * Note that XSCALE_CPAR shares bits with VECSTRIDE.