[Qemu-devel] [PULL 49/65] tcg/i386: Rely on undefined/undocumented behaviour of BSF/BSR

[Richard Henderson <rth@twiddle.net>]

The ISA manual documents the output is undefined if the input was zero.

However, we document in target-i386 that the behavior of real silicon
is to preserve the contents of the output register.  We also mention
that there are real applications that depend on this.  That this is
baked into silicon is mentioned as a potential cause for some false
sharing behaviour wrt lzcnt/tzcnt.

Taking advantage of this allows us to save 2 insns in the normal case,
and 4 insns for i686 emulating a 64-bit clz.

Signed-off-by: Richard Henderson <rth@twiddle.net>
---
 tcg/i386/tcg-target.inc.c | 35 ++++++++++++++++++++++-------------
 1 file changed, 22 insertions(+), 13 deletions(-)

diff --git a/tcg/i386/tcg-target.inc.c b/tcg/i386/tcg-target.inc.c
index 3ed8cd1..3650340 100644
--- a/tcg/i386/tcg-target.inc.c
+++ b/tcg/i386/tcg-target.inc.c
@@ -1146,9 +1146,12 @@ static void tcg_out_ctz(TCGContext *s, int rexw, TCGReg dest, TCGReg arg1,
         tcg_debug_assert(arg2 == (rexw ? 64 : 32));
         tcg_out_modrm(s, OPC_TZCNT + rexw, dest, arg1);
     } else {
-        tcg_debug_assert(dest != arg2);
+        /* ??? The manual says that the output is undefined when the
+           input is zero, but real hardware leaves it unchanged.  As
+           noted in target-i386/translate.c, real programs depend on
+           this -- now we are one more of those.  */
+        tcg_debug_assert(dest == arg2);
         tcg_out_modrm(s, OPC_BSF + rexw, dest, arg1);
-        tcg_out_cmov(s, TCG_COND_EQ, rexw, dest, arg2);
     }
 }
 
@@ -1161,20 +1164,26 @@ static void tcg_out_clz(TCGContext *s, int rexw, TCGReg dest, TCGReg arg1,
             tcg_debug_assert(arg2 == (rexw ? 64 : 32));
         } else {
             tcg_debug_assert(dest != arg2);
+            /* LZCNT sets C if the input was zero.  */
             tcg_out_cmov(s, TCG_COND_LTU, rexw, dest, arg2);
         }
     } else {
-        tcg_debug_assert(!const_a2);
-        tcg_debug_assert(dest != arg1);
-        tcg_debug_assert(dest != arg2);
+        TCGType type = rexw ? TCG_TYPE_I64: TCG_TYPE_I32;
+        TCGArg rev = rexw ? 63 : 31;
 
-        /* Recall that the output of BSR is the index not the count.  */
+        /* Recall that the output of BSR is the index not the count.
+           Therefore we must adjust the result by ^ (SIZE-1).  In some
+           cases below, we prefer an extra XOR to a JMP.  */
+        /* ??? See the comment in tcg_out_ctz re BSF.  */
+        if (const_a2) {
+            tcg_debug_assert(dest != arg1);
+            tcg_out_movi(s, type, dest, arg2 ^ rev);
+        } else {
+            tcg_debug_assert(dest == arg2);
+            tgen_arithi(s, ARITH_XOR + rexw, dest, rev, 0);
+        }
         tcg_out_modrm(s, OPC_BSR + rexw, dest, arg1);
-        tgen_arithi(s, ARITH_XOR + rexw, dest, rexw ? 63 : 31, 0);
-
-        /* Since we have destroyed the flags from BSR, we have to re-test.  */
-        tcg_out_cmp(s, arg1, 0, 1, rexw);
-        tcg_out_cmov(s, TCG_COND_EQ, rexw, dest, arg2);
+        tgen_arithi(s, ARITH_XOR + rexw, dest, rev, 0);
     }
 }
 
@@ -2443,7 +2452,7 @@ static const TCGTargetOpDef *tcg_target_op_def(TCGOpcode op)
     case INDEX_op_ctz_i64:
         {
             static const TCGTargetOpDef ctz[2] = {
-                { .args_ct_str = { "&r", "r", "r" } },
+                { .args_ct_str = { "r", "r", "0" } },
                 { .args_ct_str = { "&r", "r", "rW" } },
             };
             return &ctz[have_bmi1];
@@ -2452,7 +2461,7 @@ static const TCGTargetOpDef *tcg_target_op_def(TCGOpcode op)
     case INDEX_op_clz_i64:
         {
             static const TCGTargetOpDef clz[2] = {
-                { .args_ct_str = { "&r", "r", "r" } },
+                { .args_ct_str = { "&r", "r", "0i" } },
                 { .args_ct_str = { "&r", "r", "rW" } },
             };
             return &clz[have_lzcnt];
-- 
2.9.3