Re: [PATCH 7/7] tcg/ppc: Use prefixed instructions for tcg_out_goto_tb

["Nicholas Piggin" <npiggin@gmail.com>]

On Mon Aug 7, 2023 at 5:29 PM AEST, Nicholas Piggin wrote:
> On Mon Aug 7, 2023 at 12:13 AM AEST, Richard Henderson wrote:
> > On 8/6/23 05:55, Nicholas Piggin wrote:
> > > On Sat Aug 5, 2023 at 7:33 AM AEST, Richard Henderson wrote:
> > >> When a direct branch is out of range, we can load the destination for
> > >> the indirect branch using PLA (for 16GB worth of buffer) and PLD from
> > >> the TranslationBlock for everything larger.
> > >>
> > >> This means the patch affects exactly one instruction: B (plus filler),
> > >> PLA or PLD.  Which means we can update and execute the patch atomically.
> > >>
> > >> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> > >> ---
> > >>   tcg/ppc/tcg-target.c.inc | 76 ++++++++++++++++++++++++++++++----------
> > >>   1 file changed, 58 insertions(+), 18 deletions(-)
> > >>
> > >> diff --git a/tcg/ppc/tcg-target.c.inc b/tcg/ppc/tcg-target.c.inc
> > >> index 5b243b2353..47c71bb5f2 100644
> > >> --- a/tcg/ppc/tcg-target.c.inc
> > >> +++ b/tcg/ppc/tcg-target.c.inc
> > >> @@ -2642,31 +2642,41 @@ static void tcg_out_goto_tb(TCGContext *s, int which)
> > >>       uintptr_t ptr = get_jmp_target_addr(s, which);
> > >>   
> > >>       if (USE_REG_TB) {
> > >> +        /*
> > >> +         * With REG_TB, we must always use indirect branching,
> > >> +         * so that the branch destination and TCG_REG_TB match.
> > >> +         */
> > >>           ptrdiff_t offset = tcg_tbrel_diff(s, (void *)ptr);
> > >>           tcg_out_mem_long(s, LD, LDX, TCG_REG_TB, TCG_REG_TB, offset);
> > >> -
> > >> -        /* TODO: Use direct branches when possible. */
> > >> -        set_jmp_insn_offset(s, which);
> > >>           tcg_out32(s, MTSPR | RS(TCG_REG_TB) | CTR);
> > >> -
> > >>           tcg_out32(s, BCCTR | BO_ALWAYS);
> > >>   
> > >>           /* For the unlinked case, need to reset TCG_REG_TB.  */
> > >>           set_jmp_reset_offset(s, which);
> > >>           tcg_out_mem_long(s, ADDI, ADD, TCG_REG_TB, TCG_REG_TB,
> > >>                            -tcg_current_code_size(s));
> > >> +        return;
> > >> +    }
> > >> +
> > >> +    if (have_isa_3_10) {
> > >> +        /* Align, so that we can patch 8 bytes atomically. */
> > >> +        if ((uintptr_t)s->code_ptr & 7) {
> > >> +            tcg_out32(s, NOP);
> > >> +        }
> > >> +        set_jmp_insn_offset(s, which);
> > >> +        /* Direct branch will be patched by tb_target_set_jmp_target. */
> > >> +        tcg_out_mls_d(s, ADDI, TCG_REG_TMP1, 0, 0, 1);
> > >>       } else {
> > >>           /* Direct branch will be patched by tb_target_set_jmp_target. */
> > >> -        set_jmp_insn_offset(s, which);
> > >> -        tcg_out32(s, NOP);
> > >> -
> > >> +        tcg_out32(s, B);
> > >>           /* When branch is out of range, fall through to indirect. */
> > >>           tcg_out_movi(s, TCG_TYPE_PTR, TCG_REG_TMP1, ptr - (int16_t)ptr);
> > >>           tcg_out_ld(s, TCG_TYPE_PTR, TCG_REG_TMP1, TCG_REG_TMP1, (int16_t)ptr);
> > >> -        tcg_out32(s, MTSPR | RS(TCG_REG_TMP1) | CTR);
> > >> -        tcg_out32(s, BCCTR | BO_ALWAYS);
> > >> -        set_jmp_reset_offset(s, which);
> > >>       }
> > >> +
> > >> +    tcg_out32(s, MTSPR | RS(TCG_REG_TMP1) | CTR);
> > >> +    tcg_out32(s, BCCTR | BO_ALWAYS);
> > >> +    set_jmp_reset_offset(s, which);
> > >>   }
> > >>   
> > >>   void tb_target_set_jmp_target(const TranslationBlock *tb, int n,
> > >> @@ -2674,20 +2684,50 @@ void tb_target_set_jmp_target(const TranslationBlock *tb, int n,
> > >>   {
> > >>       uintptr_t addr = tb->jmp_target_addr[n];
> > >>       intptr_t diff = addr - jmp_rx;
> > >> -    tcg_insn_unit insn;
> > >>   
> > >>       if (USE_REG_TB) {
> > >>           return;
> > >>       }
> > >>   
> > >> -    if (in_range_b(diff)) {
> > >> -        insn = B | (diff & 0x3fffffc);
> > >> -    } else {
> > >> -        insn = NOP;
> > >> -    }
> > >> +    if (have_isa_3_10) {
> > >> +        tcg_insn_unit insn1, insn2;
> > >> +        uint64_t pair;
> > >>   
> > >> -    qatomic_set((uint32_t *)jmp_rw, insn);
> > >> -    flush_idcache_range(jmp_rx, jmp_rw, 4);
> > >> +        if (in_range_b(diff)) {
> > >> +            insn1 = B | (diff & 0x3fffffc);
> > >> +            insn2 = NOP;
> > >> +        } else if (diff == sextract64(diff, 0, 34)) {
> > >> +            /* PLA tmp1, diff */
> > >> +            insn1 = OPCD(1) | (2 << 24) | (1 << 20) | ((diff >> 16) & 0x3ffff);
> > >> +            insn2 = ADDI | TAI(TCG_REG_TMP1, 0, diff);
> > >> +        } else {
> > >> +            addr = (uintptr_t)&tb->jmp_target_addr[n];
> > >> +            diff = addr - jmp_rx;
> > >> +            tcg_debug_assert(diff == sextract64(diff, 0, 34));
> > >> +            /* PLD tmp1, diff */
> > >> +            insn1 = OPCD(1) | (1 << 20) | ((diff >> 16) & 0x3ffff);
> > >> +            insn2 = PLD | TAI(TCG_REG_TMP1, 0, diff);
> > >> +        }
> > > 
> > > B is a "patch class" word instruction as per CMODX in the ISA, which may
> > > be patched to/from other instructions without a flush+isync sequence
> > > betwen. So that part is okay, at least if you were just patching the B
> > > word. But patching between the PLA and PLD I don't think is kosher per
> > > ISA.
> > > 
> > > I struggle a bit with this part of the ISA, particularly with prefix
> > > instructions (it only talks about patching 4 bytes at a time).
> > > 
> > > If we patch something it has to go through a patch instruction, which
> > > is a direct branch, trap, or nop. I think that makes this non-trivial.
> > > 
> > > It could work if you only patched between B and PLD. B->PLD would have
> > > to patch the suffix word first, possibly with an interleaving sync, and
> > > then the prefix. PLD->B could just patch the B word.
> > > 
> > > How much would losing the PLA hurt?
> >
> > Really?  I can't imagine how some icache would see a torn prefixed insn given an atomic 
> > store (CMODX talks about prefixed instructions which "may be unaligned" -- but what if 
> > they are not?).
> >
> > But if patching an aligned prefixed insn isn't allowed, I would patch between B and NOP, 
> > leave the PLD alone on the fall-through path, and drop the PLA.
>
> Hmm, even patching two different offset B instructions in some sequence
> with a PLD would go against the ISA, because PLD is not a patch class
> instruction. The only case you can patch those has to have a sequence of
> exactly two instruction values. So I think you need the B first, and you
> can patch that between any offset of B or a NOP to fall through to PLD.
> NOP and B are both patch class, so any sequence of them is allowed.

Here is an incremental diff that hopefully solves those issues. Sadly,
by removing the nice PLA patching :( But it seems to be around or close
to original performance on my test.

Feel free to squash it in or take inspiration (or dispute it).

Thanks,
Nick

---
diff --git a/tcg/ppc/tcg-target.c.inc b/tcg/ppc/tcg-target.c.inc
index 47c71bb5f2..569c2e3647 100644
--- a/tcg/ppc/tcg-target.c.inc
+++ b/tcg/ppc/tcg-target.c.inc
@@ -748,6 +748,11 @@ static void tcg_out_prefix_align(TCGContext *s)
     }
 }
 
+static ptrdiff_t tcg_pcrel_diff_for_prefix(TCGContext *s, const void *target)
+{
+    return tcg_pcrel_diff(s, target) - (tcg_out_need_prefix_align(s) ? 4 : 0);
+}
+
 /* Output Type 00 Prefix - 8-Byte Load/Store Form (8LS:D) */
 static void tcg_out_8ls_d(TCGContext *s, tcg_insn_unit opc, unsigned rt,
                           unsigned ra, tcg_target_long imm, bool r)
@@ -1072,8 +1077,7 @@ static void tcg_out_movi_int(TCGContext *s, TCGType type, TCGReg ret,
             return;
         }
 
-        tmp = tcg_out_need_prefix_align(s) * 4;
-        tmp = tcg_pcrel_diff(s, (void *)arg) - tmp;
+        tmp = tcg_pcrel_diff_for_prefix(s, (void *)arg);
         if (tmp == sextract64(tmp, 0, 34)) {
             /* pla ret,value = paddi ret,0,value,1 */
             tcg_out_mls_d(s, ADDI, ret, 0, tmp, 1);
@@ -2658,18 +2662,19 @@ static void tcg_out_goto_tb(TCGContext *s, int which)
         return;
     }
 
+    /* Direct branch will be patched by tb_target_set_jmp_target. */
+    set_jmp_insn_offset(s, which);
+    tcg_out32(s, NOP);
+
+    /* When branch is out of range, fall through to indirect. */
     if (have_isa_3_10) {
-        /* Align, so that we can patch 8 bytes atomically. */
-        if ((uintptr_t)s->code_ptr & 7) {
-            tcg_out32(s, NOP);
-        }
-        set_jmp_insn_offset(s, which);
-        /* Direct branch will be patched by tb_target_set_jmp_target. */
-        tcg_out_mls_d(s, ADDI, TCG_REG_TMP1, 0, 0, 1);
+        ptrdiff_t offset = tcg_pcrel_diff_for_prefix(s, (void *)ptr);
+        /*
+         * Would be nice to use PLA if offset is in range,
+         * but CMODX rules make that difficult.
+         */
+        tcg_out_8ls_d(s, PLD, TCG_REG_TMP1, 0, offset, 1);
     } else {
-        /* Direct branch will be patched by tb_target_set_jmp_target. */
-        tcg_out32(s, B);
-        /* When branch is out of range, fall through to indirect. */
         tcg_out_movi(s, TCG_TYPE_PTR, TCG_REG_TMP1, ptr - (int16_t)ptr);
         tcg_out_ld(s, TCG_TYPE_PTR, TCG_REG_TMP1, TCG_REG_TMP1, (int16_t)ptr);
     }
@@ -2684,50 +2689,19 @@ void tb_target_set_jmp_target(const TranslationBlock *tb, int n,
 {
     uintptr_t addr = tb->jmp_target_addr[n];
     intptr_t diff = addr - jmp_rx;
+    tcg_insn_unit insn;
 
     if (USE_REG_TB) {
         return;
     }
 
-    if (have_isa_3_10) {
-        tcg_insn_unit insn1, insn2;
-        uint64_t pair;
-
-        if (in_range_b(diff)) {
-            insn1 = B | (diff & 0x3fffffc);
-            insn2 = NOP;
-        } else if (diff == sextract64(diff, 0, 34)) {
-            /* PLA tmp1, diff */
-            insn1 = OPCD(1) | (2 << 24) | (1 << 20) | ((diff >> 16) & 0x3ffff);
-            insn2 = ADDI | TAI(TCG_REG_TMP1, 0, diff);
-        } else {
-            addr = (uintptr_t)&tb->jmp_target_addr[n];
-            diff = addr - jmp_rx;
-            tcg_debug_assert(diff == sextract64(diff, 0, 34));
-            /* PLD tmp1, diff */
-            insn1 = OPCD(1) | (1 << 20) | ((diff >> 16) & 0x3ffff);
-            insn2 = PLD | TAI(TCG_REG_TMP1, 0, diff);
-        }
-
-        if (HOST_BIG_ENDIAN) {
-            pair = ((uint64_t)insn1) << 32 | insn2;
-        } else {
-            pair = ((uint64_t)insn2) << 32 | insn1;
-        }
-
-        qatomic_set((uint64_t *)jmp_rw, pair);
-        flush_idcache_range(jmp_rx, jmp_rw, 8);
+    if (in_range_b(diff)) {
+        insn = B | (diff & 0x3fffffc);
     } else {
-        tcg_insn_unit insn;
-
-        if (in_range_b(diff)) {
-            insn = B | (diff & 0x3fffffc);
-        } else {
-            insn = NOP;
-        }
-        qatomic_set((uint32_t *)jmp_rw, insn);
-        flush_idcache_range(jmp_rx, jmp_rw, 4);
+        insn = NOP;
     }
+    qatomic_set((uint32_t *)jmp_rw, insn);
+    flush_idcache_range(jmp_rx, jmp_rw, 4);
 }
 
 static void tcg_out_op(TCGContext *s, TCGOpcode opc,