[Qemu-devel] [PATCH v3] TCG x86: implement lzcnt emulation

[Andre Przywara]

lzcnt is a AMD Phenom/Barcelona added instruction returning the
number of leading zero bits in a word.
As this is similar to the "bsr" instruction, reuse the existing
code. There need to be some more changes, though, as lzcnt always
returns a valid value (in opposite to bsr, which has a special
case when the operand is 0).
lzcnt is guarded by the ABM CPUID bit (Fn8000_0001:ECX_5).

Signed-off-by: Andre Przywara <address@hidden>
---
 target-i386/helper.h    |    1 +
 target-i386/op_helper.c |   14 ++++++++++++--
 target-i386/translate.c |   37 +++++++++++++++++++++++++------------
 3 files changed, 38 insertions(+), 14 deletions(-)

diff --git a/target-i386/helper.h b/target-i386/helper.h
index ca953f4..6b518ad 100644
--- a/target-i386/helper.h
+++ b/target-i386/helper.h
@@ -193,6 +193,7 @@ DEF_HELPER_2(fxsave, void, tl, int)
 DEF_HELPER_2(fxrstor, void, tl, int)
 DEF_HELPER_1(bsf, tl, tl)
 DEF_HELPER_1(bsr, tl, tl)
+DEF_HELPER_2(lzcnt, tl, tl, int)
 
 /* MMX/SSE */
 
diff --git a/target-i386/op_helper.c b/target-i386/op_helper.c
index 26fe612..5eea322 100644
--- a/target-i386/op_helper.c
+++ b/target-i386/op_helper.c
@@ -5479,11 +5479,14 @@ target_ulong helper_bsf(target_ulong t0)
     return count;
 }
 
-target_ulong helper_bsr(target_ulong t0)
+target_ulong helper_lzcnt(target_ulong t0, int wordsize)
 {
     int count;
     target_ulong res, mask;
-    
+
+    if (wordsize > 0 && t0 == 0) {
+        return wordsize;
+    }
     res = t0;
     count = TARGET_LONG_BITS - 1;
     mask = (target_ulong)1 << (TARGET_LONG_BITS - 1);
@@ -5491,9 +5494,16 @@ target_ulong helper_bsr(target_ulong t0)
         count--;
         res <<= 1;
     }
+    if (wordsize > 0) {
+        return wordsize - 1 - count;
+    }
     return count;
 }
 
+target_ulong helper_bsr(target_ulong t0)
+{
+       return helper_lzcnt(t0, 0);
+}
 
 static int compute_all_eflags(void)
 {
diff --git a/target-i386/translate.c b/target-i386/translate.c
index 2511943..64bc0a3 100644
--- a/target-i386/translate.c
+++ b/target-i386/translate.c
@@ -6573,23 +6573,36 @@ static target_ulong disas_insn(DisasContext *s, 
target_ulong pc_start)
             ot = dflag + OT_WORD;
             modrm = ldub_code(s->pc++);
             reg = ((modrm >> 3) & 7) | rex_r;
-            gen_ldst_modrm(s, modrm, ot, OR_TMP0, 0);
+            gen_ldst_modrm(s,modrm, ot, OR_TMP0, 0);
             gen_extu(ot, cpu_T[0]);
-            label1 = gen_new_label();
-            tcg_gen_movi_tl(cpu_cc_dst, 0);
             t0 = tcg_temp_local_new();
             tcg_gen_mov_tl(t0, cpu_T[0]);
-            tcg_gen_brcondi_tl(TCG_COND_EQ, t0, 0, label1);
-            if (b & 1) {
-                gen_helper_bsr(cpu_T[0], t0);
+            if ((b & 1) && (prefixes & PREFIX_REPZ) &&
+                (s->cpuid_ext3_features & CPUID_EXT3_ABM)) {
+                switch(ot) {
+                case OT_WORD: gen_helper_lzcnt(cpu_T[0], t0,
+                    tcg_const_i32(16)); break;
+                case OT_LONG: gen_helper_lzcnt(cpu_T[0], t0,
+                    tcg_const_i32(32)); break;
+                case OT_QUAD: gen_helper_lzcnt(cpu_T[0], t0,
+                    tcg_const_i32(64)); break;
+                }
+                gen_op_mov_reg_T0(ot, reg);
             } else {
-                gen_helper_bsf(cpu_T[0], t0);
+                label1 = gen_new_label();
+                tcg_gen_movi_tl(cpu_cc_dst, 0);
+                tcg_gen_brcondi_tl(TCG_COND_EQ, t0, 0, label1);
+                if (b & 1) {
+                    gen_helper_bsr(cpu_T[0], t0);
+                } else {
+                    gen_helper_bsf(cpu_T[0], t0);
+                }
+                gen_op_mov_reg_T0(ot, reg);
+                tcg_gen_movi_tl(cpu_cc_dst, 1);
+                gen_set_label(label1);
+                tcg_gen_discard_tl(cpu_cc_src);
+                s->cc_op = CC_OP_LOGICB + ot;
             }
-            gen_op_mov_reg_T0(ot, reg);
-            tcg_gen_movi_tl(cpu_cc_dst, 1);
-            gen_set_label(label1);
-            tcg_gen_discard_tl(cpu_cc_src);
-            s->cc_op = CC_OP_LOGICB + ot;
             tcg_temp_free(t0);
         }
         break;
-- 
1.6.4