[PATCH] riscv: avoid vle8ff in vector strcmp loop

[daichengrong]

Limit vl by the minimum distance from the input pointers to the next 4K
boundary, and use normal vle8.v loads in the main loop.  This keeps the
loads page-safe while avoiding the vl update dependency from vle8ff.v.

Signed-off-by: daichengrong <daichengrong@iscas.ac.cn>
---
 sysdeps/riscv/rvv/strcmp.S | 110 +++++++++++++++++++++----------------
 1 file changed, 64 insertions(+), 46 deletions(-)

diff --git a/sysdeps/riscv/rvv/strcmp.S b/sysdeps/riscv/rvv/strcmp.S
index a2f6865581..d32d8f490a 100644
--- a/sysdeps/riscv/rvv/strcmp.S
+++ b/sysdeps/riscv/rvv/strcmp.S
@@ -35,59 +35,77 @@
 #define vstr1 v0
 #define vstr2 v8
 #define vmask1 v16
-#define vmask2 v17
+#define vmask2 v24
 
 ENTRY (STRCMP)
 .option push
 .option arch, +v
-    /* lmul=1 */
-L(Loop):
-    vsetvli ivl, zero, e8, m1, ta, ma
-    vle8ff.v vstr1, (str1)
-    /* Check if vstr1[i] == 0 */
-    vmseq.vx vmask1, vstr1, zero
-
-    vle8ff.v vstr2, (str2)
-    /* Check if vstr1[i] != vstr2[i] */
-    vmsne.vv vmask2, vstr1, vstr2
-
-    /* Find the index x for vstr1[x] == 0 */
-    vfirst.m temp1, vmask1
-    /* Find the index x for vstr1[x] != vstr2[x] */
-    vfirst.m temp2, vmask2
-
-    bgez temp1, L(check1)
-    bgez temp2, L(check2)
-
-    /* Get the current vl updated by vle8ff. */
-    csrr ivl, vl
-    add str1, str1, ivl
-    add str2, str2, ivl
-    j L(Loop)
-
-    /* temp1 >= 0 */
-L(check1):
-    bltz temp2, L(return_at_nul)
-    blt temp2, temp1, L(check2)
-L(return_at_nul):
-    /* temp2 < 0 */
-    /* temp2 >= 0 && temp1 < temp2 */
-    add str1, str1, temp1
-    add str2, str2, temp1
-    lbu temp1, 0(str1)
-    lbu temp2, 0(str2)
-    sub result, temp1, temp2
+    li          t0, 4095
+    li          t3, 4096
+L(loop):
+    and         t1, str1, t0      /* t1 = str1 offset in 4K page */
+    and         t2, str2, t0      /* t2 = str2 offset in 4K page */
+    sub         t4, t3, t1        /* t4 = bytes from str1 to page end */
+    sub         t5, t3, t2        /* t5 = bytes from str2 to page end */
+    /*
+     * t3 = min(dist1, dist2)
+     */
+    bltu        t5, t4, 1f
+    mv          t5, t4
+1:
+    /*
+     * Set vl to min(dist, VLMAX).
+     *
+     * If far from page boundary, vl = VLMAX.
+     * If close to page boundary, vl = dist.
+     */
+
+    vsetvli     ivl, t5, e8, m4, ta, ma
+    /*
+     * This load will not cross a 4K boundary for either pointer.
+     */
+    vle8.v      vstr1, (str1)
+    vle8.v      vstr2, (str2)
+
+    /*
+     * str1 == 0 means string ended.
+     * str1 != str2 means strcmp result is decided.
+     */
+    vmseq.vx    vmask1, vstr1, zero
+    vmsne.vv    vmask2, vstr1, vstr2
+
+    vfirst.m    temp2, vmask2
+    vfirst.m    temp1, vmask1
+
+    bgez        temp2, L(check_nul)
+    bgez        temp1, L(ret)
+
+    /*
+     * No NUL and no difference in this chunk.
+     * Advance by actual vl.
+     *
+     * If this was a page-boundary-limited load, one of str1/str2 now
+     * reaches the next 4K boundary. The next loop recomputes distance.
+     */
+    add         str1, str1, ivl
+    add         str2, str2, ivl
+    j           L(loop)
+
+L(check_nul):
+    bltz        temp1, L(found)
+    blt         temp1, temp2, L(ret)
+L(found):
+    add         str1, str1, temp2
+    add         str2, str2, temp2
+    lbu         temp1, 0(str1)
+    lbu         temp2, 0(str2)
+    sub         result, temp1, temp2
     ret
 
-    /* temp1 < 0 */
-    /* temp2 >= 0 */
-L(check2):
-    add str1, str1, temp2
-    add str2, str2, temp2
-    lbu temp1, 0(str1)
-    lbu temp2, 0(str2)
-    sub result, temp1, temp2
+L(ret):
+    li          result, 0
     ret
+
 .option pop
 END (STRCMP)
 libc_hidden_builtin_def (strcmp)
-- 
2.25.1