Re: [PATCH] target/riscv: Use a direct cast for better performance

[Daniel Henrique Barboza]

On 10/7/23 06:02, Richard W.M. Jones wrote:

RISCV_CPU(cs) uses a checked cast.  When QOM cast debugging is enabled
this adds about 5% total overhead when emulating RV64 on x86-64 host.

Using a RISC-V guest with 16 vCPUs, 16 GB of guest RAM, virtio-blk
disk.  The guest has a copy of the qemu source tree.  The test
involves compiling the qemu source tree with 'make clean; time make -j16'.

Before making this change the compile step took 449 & 447 seconds over
two consecutive runs.

After making this change, 428 & 422 seconds.

The saving is about 5%.

Thanks: Paolo Bonzini
Signed-off-by: Richard W.M. Jones <rjo...@redhat.com>
---

Reviewed-by: Daniel Henrique Barboza <dbarb...@ventanamicro.com>

  target/riscv/cpu_helper.c | 6 +++++-
  1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
index 3a02079290..6174d99fb2 100644
--- a/target/riscv/cpu_helper.c
+++ b/target/riscv/cpu_helper.c
@@ -66,7 +66,11 @@ void cpu_get_tb_cpu_state(CPURISCVState *env, vaddr *pc,
                            uint64_t *cs_base, uint32_t *pflags)
  {
      CPUState *cs = env_cpu(env);
-    RISCVCPU *cpu = RISCV_CPU(cs);
+    /*
+     * Using the checked cast RISCV_CPU(cs) imposes ~ 5% overhead when
+     * qemu cast debugging is enabled, so use a direct cast instead.
+     */
+    RISCVCPU *cpu = (RISCVCPU *)cs;
      RISCVExtStatus fs, vs;
      uint32_t flags = 0;