s390x: XC instruction: clear in 8-byte increments if possible

The XC instruction is frequently executed in many programs, mainly for
clearing memory.  It can target from 1 to 256 bytes.  If the size is
constant and XC is actually used for clearing memory, Valgrind implements
it as a byte-wise loop and rolls out the loop for <= 8 bytes.

Instead of clearing byte-wise, it is more efficient to clear in 64-bit
increments, so do this for sizes >= 8 bytes.  Roll out the loop for up to
32 bytes.  Overall, this reduces the number of insns by a few percent and
provides a slight performance improvement for some programs.

diff --git a/VEX/priv/guest_s390_toIR.c b/VEX/priv/guest_s390_toIR.c
index 39356e088f914c78b0dc36155732908f3e20bbb9..11dda41ef5fca3535726f0862ef8e628911cb1df 100644 (file)
--- a/VEX/priv/guest_s390_toIR.c
+++ b/VEX/priv/guest_s390_toIR.c
@@ -13615,36 +13615,36 @@ s390_irgen_XC(UChar length, IRTemp start1, IRTemp start2)
 static void
 s390_irgen_XC_sameloc(UChar length, UChar b, UShort d)
 {
-   IRTemp counter = newTemp(Ity_I32);
    IRTemp start = newTemp(Ity_I64);
-   IRTemp addr  = newTemp(Ity_I64);
-
    assign(start,
           binop(Iop_Add64, mkU64(d), b != 0 ? get_gpr_dw0(b) : mkU64(0)));
 
-   if (length < 8) {
-      UInt i;
-
-      for (i = 0; i <= length; ++i) {
+   if (length < 7) {
+      for (UInt i = 0; i <= length; ++i) {
          store(binop(Iop_Add64, mkexpr(start), mkU64(i)), mkU8(0));
       }
    } else {
-     assign(counter, get_counter_w0());
-
-     assign(addr, binop(Iop_Add64, mkexpr(start),
-                        unop(Iop_32Uto64, mkexpr(counter))));
-
-     store(mkexpr(addr), mkU8(0));
-
-     /* Check for end of field */
-     put_counter_w0(binop(Iop_Add32, mkexpr(counter), mkU32(1)));
-     iterate_if(binop(Iop_CmpNE32, mkexpr(counter), mkU32(length)));
-
-     /* Reset counter */
-     put_counter_dw0(mkU64(0));
+      if (length < 32) {
+         for (UInt i = 0; i <= length - 7; i += 8) {
+            store(binop(Iop_Add64, mkexpr(start), mkU64(i)), mkU64(0));
+         }
+      } else {
+         IRTemp counter = newTemp(Ity_I64);
+         assign(counter, get_counter_dw0());
+         store(binop(Iop_Add64, mkexpr(start), mkexpr(counter)), mkU64(0));
+         put_counter_dw0(binop(Iop_Add64, mkexpr(counter), mkU64(8)));
+         iterate_if(binop(Iop_CmpLE64U, mkexpr(counter), mkU64(length - 15)));
+
+         /* Reset counter */
+         put_counter_dw0(mkU64(0));
+      }
+      /* Clear the remaining bytes with backward overlap */
+      if ((length + 1) % 8 != 0) {
+         store(binop(Iop_Add64, mkexpr(start), mkU64(length - 7)), mkU64(0));
+      }
    }
 
-   s390_cc_thunk_put1(S390_CC_OP_BITWISE, mktemp(Ity_I32, mkU32(0)), False);
+   s390_cc_set_val(0);
 
    if (UNLIKELY(vex_traceflags & VEX_TRACE_FE))
       s390_disasm(ENC3(MNM, UDLB, UDXB), "xc", d, length, b, d, 0, b);