Re: [RFC 1/2] target/riscv: rvv: reduce the overhead for simple RISC-V vector unit-stride loads and stores

[Daniel Henrique Barboza]

On 7/27/24 4:13 AM, Richard Henderson wrote:

On 7/18/24 01:30, Paolo Savini wrote:

From: Helene CHELIN <helene.che...@embecosm.com>

This patch improves the performance of the emulation of the RVV unit-stride
loads and stores in the following cases:

- when the data being loaded/stored per iteration amounts to 8 bytes or less.
- when the vector length is 16 bytes (VLEN=128) and there's no grouping of the
   vector registers (LMUL=1).

The optimization consists of avoiding the overhead of probing the RAM of the
host machine and doing a loop load/store on the input data grouped in chunks
of as many bytes as possible (8,4,2,1 bytes).

Co-authored-by: Helene CHELIN <helene.che...@embecosm.com>
Co-authored-by: Paolo Savini <paolo.sav...@embecosm.com>

Signed-off-by: Helene CHELIN <helene.che...@embecosm.com>
---
  target/riscv/vector_helper.c | 46 ++++++++++++++++++++++++++++++++++++
  1 file changed, 46 insertions(+)

diff --git a/target/riscv/vector_helper.c b/target/riscv/vector_helper.c
index 29849a8b66..4b444c6bc5 100644
--- a/target/riscv/vector_helper.c
+++ b/target/riscv/vector_helper.c
@@ -633,6 +633,52 @@ vext_ldst_us(void *vd, target_ulong base, CPURISCVState 
*env, uint32_t desc,
      VSTART_CHECK_EARLY_EXIT(env);
+    /* For data sizes <= 64 bits and for LMUL=1 with VLEN=128 bits we get a
+     * better performance by doing a simple simulation of the load/store
+     * without the overhead of prodding the host RAM */
+    if ((nf == 1) && ((evl << log2_esz) <= 8 ||
+    ((vext_lmul(desc) == 0) && (simd_maxsz(desc) == 16)))) {
+
+    uint32_t evl_b = evl << log2_esz;
+
+        for (uint32_t j = env->vstart; j < evl_b;) {
+        addr = base + j;
+            if ((evl_b - j) >= 8) {
+                if (is_load)
+                    lde_d_tlb(env, adjust_addr(env, addr), j, vd, ra);
+                else
+                    ste_d_tlb(env, adjust_addr(env, addr), j, vd, ra);
+                j += 8;
+            }
+            else if ((evl_b - j) >= 4) {
+                if (is_load)
+                    lde_w_tlb(env, adjust_addr(env, addr), j, vd, ra);
+                else
+                    ste_w_tlb(env, adjust_addr(env, addr), j, vd, ra);
+                j += 4;
+            }
+            else if ((evl_b - j) >= 2) {
+                if (is_load)
+                    lde_h_tlb(env, adjust_addr(env, addr), j, vd, ra);
+                else
+                    ste_h_tlb(env, adjust_addr(env, addr), j, vd, ra);
+                j += 2;
+            }
+            else {
+                if (is_load)
+                    lde_b_tlb(env, adjust_addr(env, addr), j, vd, ra);
+                else
+                    ste_b_tlb(env, adjust_addr(env, addr), j, vd, ra);
+                j += 1;
+            }
+        }

For system mode, this performs the tlb lookup N times, and so will not be an 
improvement.

I believe we can wrap this up in an "#ifdef CONFIG_USER_ONLY" block to allow
linux-user mode to benefit from it. We would still need to take care of the
host endianess though.


Thanks,

Daniel

This will not work on a big-endian host.


r~