target/xtensa: rework zero overhead loops implementation

Don't invalidate TB with the end of zero overhead loop when LBEG or LEND
change. Instead encode the distance from the start of the page where the
TB starts to the LEND in the TB cs_base and generate loopback code when
the next PC matches encoded LEND. Distance to a destination within the
same page and up to a maximum instruction length into the next page is
encoded literally, otherwise it's zero. The distance from LEND to LBEG
is also encoded in the cs_base: it's encoded literally when less than
256 or as 0 otherwise. This allows for TB chaining for the loopback
branch at the end of a loop for the most common loop sizes.

With this change the resulting emulation speed is about 10% higher in
softmmu mode on uClibc-ng and LTP tests. Emulation speed in linux
user mode is a few percent lower because there's no direct TB chaining
between different memory pages. Testing with lower limit on direct TB
chaining range shows gradual slowdown to ~15% for the block size of 64
bytes and ~50% for the block size of 32 bytes.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

1 files changed, 16 insertions, 37 deletions

diff --git a/target/xtensa/translate.c b/target/xtensa/translate.c
index 46e1338448..a435d9c36c 100644
--- a/target/xtensa/translate.c
+++ b/target/xtensa/translate.c
@@ -53,7 +53,7 @@ struct DisasContext {
     uint32_t pc;
     int cring;
     int ring;
-    uint32_t lbeg;
+    uint32_t lbeg_off;
     uint32_t lend;
 
     bool sar_5bit;
@@ -390,11 +390,9 @@ static void gen_jump(DisasContext *dc, TCGv dest)
 static void gen_jumpi(DisasContext *dc, uint32_t dest, int slot)
 {
     TCGv_i32 tmp = tcg_const_i32(dest);
-#ifndef CONFIG_USER_ONLY
     if (((dc->base.pc_first ^ dest) & TARGET_PAGE_MASK) != 0) {
         slot = -1;
     }
-#endif
     gen_jump_slot(dc, tmp, slot);
     tcg_temp_free(tmp);
 }
@@ -420,25 +418,25 @@ static void gen_callw(DisasContext *dc, int callinc, TCGv_i32 dest)
 static void gen_callwi(DisasContext *dc, int callinc, uint32_t dest, int slot)
 {
     TCGv_i32 tmp = tcg_const_i32(dest);
-#ifndef CONFIG_USER_ONLY
     if (((dc->base.pc_first ^ dest) & TARGET_PAGE_MASK) != 0) {
         slot = -1;
     }
-#endif
     gen_callw_slot(dc, callinc, tmp, slot);
     tcg_temp_free(tmp);
 }
 
 static bool gen_check_loop_end(DisasContext *dc, int slot)
 {
-    if (option_enabled(dc, XTENSA_OPTION_LOOP) &&
-            !(dc->base.tb->flags & XTENSA_TBFLAG_EXCM) &&
-            dc->base.pc_next == dc->lend) {
+    if (dc->base.pc_next == dc->lend) {
         TCGLabel *label = gen_new_label();
 
         tcg_gen_brcondi_i32(TCG_COND_EQ, cpu_SR[LCOUNT], 0, label);
         tcg_gen_subi_i32(cpu_SR[LCOUNT], cpu_SR[LCOUNT], 1);
-        gen_jumpi(dc, dc->lbeg, slot);
+        if (dc->lbeg_off) {
+            gen_jumpi(dc, dc->base.pc_next - dc->lbeg_off, slot);
+        } else {
+            gen_jump(dc, cpu_SR[LBEG]);
+        }
         gen_set_label(label);
         gen_jumpi(dc, dc->base.pc_next, -1);
         return true;
@@ -534,16 +532,6 @@ static void gen_rsr(DisasContext *dc, TCGv_i32 d, uint32_t sr)
     }
 }
 
-static void gen_wsr_lbeg(DisasContext *dc, uint32_t sr, TCGv_i32 s)
-{
-    gen_helper_wsr_lbeg(cpu_env, s);
-}
-
-static void gen_wsr_lend(DisasContext *dc, uint32_t sr, TCGv_i32 s)
-{
-    gen_helper_wsr_lend(cpu_env, s);
-}
-
 static void gen_wsr_sar(DisasContext *dc, uint32_t sr, TCGv_i32 s)
 {
     tcg_gen_andi_i32(cpu_SR[sr], s, 0x3f);
@@ -743,8 +731,6 @@ static void gen_wsr(DisasContext *dc, uint32_t sr, TCGv_i32 s)
 {
     static void (* const wsr_handler[256])(DisasContext *dc,
                                            uint32_t sr, TCGv_i32 v) = {
-        [LBEG] = gen_wsr_lbeg,
-        [LEND] = gen_wsr_lend,
         [SAR] = gen_wsr_sar,
         [BR] = gen_wsr_br,
         [LITBASE] = gen_wsr_litbase,
@@ -906,13 +892,6 @@ static void disas_xtensa_insn(CPUXtensaState *env, DisasContext *dc)
     }
 
     dc->base.pc_next = dc->pc + len;
-    if (xtensa_option_enabled(dc->config, XTENSA_OPTION_LOOP) &&
-        dc->lbeg == dc->pc &&
-        ((dc->pc ^ (dc->base.pc_next - 1)) & -dc->config->inst_fetch_width)) {
-        qemu_log_mask(LOG_GUEST_ERROR,
-                      "unaligned first instruction of a loop (pc = %08x)\n",
-                      dc->pc);
-    }
     for (i = 1; i < len; ++i) {
         b[i] = cpu_ldub_code(env, dc->pc + i);
     }
@@ -1097,8 +1076,10 @@ static void xtensa_tr_init_disas_context(DisasContextBase *dcbase,
     dc->pc = dc->base.pc_first;
     dc->ring = tb_flags & XTENSA_TBFLAG_RING_MASK;
     dc->cring = (tb_flags & XTENSA_TBFLAG_EXCM) ? 0 : dc->ring;
-    dc->lbeg = env->sregs[LBEG];
-    dc->lend = env->sregs[LEND];
+    dc->lbeg_off = (dc->base.tb->cs_base & XTENSA_CSBASE_LBEG_OFF_MASK) >>
+        XTENSA_CSBASE_LBEG_OFF_SHIFT;
+    dc->lend = (dc->base.tb->cs_base & XTENSA_CSBASE_LEND_MASK) +
+        (dc->base.pc_first & TARGET_PAGE_MASK);
     dc->debug = tb_flags & XTENSA_TBFLAG_DEBUG;
     dc->icount = tb_flags & XTENSA_TBFLAG_ICOUNT;
     dc->cpenable = (tb_flags & XTENSA_TBFLAG_CPENABLE_MASK) >>
@@ -1712,12 +1693,10 @@ static void translate_loop(DisasContext *dc, const uint32_t arg[],
                            const uint32_t par[])
 {
     uint32_t lend = arg[1];
-    TCGv_i32 tmp = tcg_const_i32(lend);
 
     tcg_gen_subi_i32(cpu_SR[LCOUNT], cpu_R[arg[0]], 1);
     tcg_gen_movi_i32(cpu_SR[LBEG], dc->base.pc_next);
-    gen_helper_wsr_lend(cpu_env, tmp);
-    tcg_temp_free(tmp);
+    tcg_gen_movi_i32(cpu_SR[LEND], lend);
 
     if (par[0] != TCG_COND_NEVER) {
         TCGLabel *label = gen_new_label();
@@ -4609,7 +4588,7 @@ static const XtensaOpcodeOps core_ops[] = {
         .translate = translate_wsr,
         .test_ill = test_ill_wsr,
         .par = (const uint32_t[]){LBEG},
-        .op_flags = XTENSA_OP_EXIT_TB_0,
+        .op_flags = XTENSA_OP_EXIT_TB_M1,
         .windowed_register_op = 0x1,
     }, {
         .name = "wsr.lcount",
@@ -4622,7 +4601,7 @@ static const XtensaOpcodeOps core_ops[] = {
         .translate = translate_wsr,
         .test_ill = test_ill_wsr,
         .par = (const uint32_t[]){LEND},
-        .op_flags = XTENSA_OP_EXIT_TB_0,
+        .op_flags = XTENSA_OP_EXIT_TB_M1,
         .windowed_register_op = 0x1,
     }, {
         .name = "wsr.litbase",
@@ -5183,7 +5162,7 @@ static const XtensaOpcodeOps core_ops[] = {
         .translate = translate_xsr,
         .test_ill = test_ill_xsr,
         .par = (const uint32_t[]){LBEG},
-        .op_flags = XTENSA_OP_EXIT_TB_0,
+        .op_flags = XTENSA_OP_EXIT_TB_M1,
         .windowed_register_op = 0x1,
     }, {
         .name = "xsr.lcount",
@@ -5196,7 +5175,7 @@ static const XtensaOpcodeOps core_ops[] = {
         .translate = translate_xsr,
         .test_ill = test_ill_xsr,
         .par = (const uint32_t[]){LEND},
-        .op_flags = XTENSA_OP_EXIT_TB_0,
+        .op_flags = XTENSA_OP_EXIT_TB_M1,
         .windowed_register_op = 0x1,
     }, {
         .name = "xsr.litbase",