[PATCH 8/8] tcg/loongarch64: Reorg goto_tb implementation

[Richard Henderson <richard.henderson@linaro.org>]

The old implementation replaces two insns, swapping between

        b       <dest>
        nop
and
        pcaddu18i tmp, <dest>
        jirl      zero, tmp, <dest> & 0xffff

There is a race condition in which a thread could be stopped at
the jirl, i.e. with the top of the address loaded, and when
restarted we have re-linked to a different TB, so that the top
half no longer matches the bottom half.

Note that while we never directly re-link to a different TB, we
can link, unlink, and link again all while the stopped thread
remains stopped.

The new implementation replaces only one insn, swapping between

        b       <dest>
and
        nop

falling through to a general-case indirect branch.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 tcg/loongarch64/tcg-target.h     |  7 +---
 tcg/loongarch64/tcg-target.c.inc | 67 ++++++++++++--------------------
 2 files changed, 26 insertions(+), 48 deletions(-)

diff --git a/tcg/loongarch64/tcg-target.h b/tcg/loongarch64/tcg-target.h
index 624fbe87ff..81548fbb09 100644
--- a/tcg/loongarch64/tcg-target.h
+++ b/tcg/loongarch64/tcg-target.h
@@ -42,11 +42,8 @@
 
 #define TCG_TARGET_INSN_UNIT_SIZE 4
 #define TCG_TARGET_NB_REGS 32
-/*
- * PCADDU18I + JIRL sequence can give 20 + 16 + 2 = 38 bits
- * signed offset, which is +/- 128 GiB.
- */
-#define MAX_CODE_GEN_BUFFER_SIZE  (128 * GiB)
+
+#define MAX_CODE_GEN_BUFFER_SIZE  ((size_t)-1)
 
 typedef enum {
     TCG_REG_ZERO,
diff --git a/tcg/loongarch64/tcg-target.c.inc b/tcg/loongarch64/tcg-target.c.inc
index 47465b8c20..f8964699eb 100644
--- a/tcg/loongarch64/tcg-target.c.inc
+++ b/tcg/loongarch64/tcg-target.c.inc
@@ -1150,37 +1150,6 @@ static void tcg_out_qemu_st(TCGContext *s, const TCGArg *args)
 #endif
 }
 
-/* LoongArch uses `andi zero, zero, 0` as NOP.  */
-#define NOP OPC_ANDI
-static void tcg_out_nop(TCGContext *s)
-{
-    tcg_out32(s, NOP);
-}
-
-void tb_target_set_jmp_target(const TranslationBlock *tb, int n,
-                              uintptr_t jmp_rx, uintptr_t jmp_rw)
-{
-    tcg_insn_unit i1, i2;
-    ptrdiff_t upper, lower;
-    uintptr_t addr = tb->jmp_target_addr[n];
-    ptrdiff_t offset = (ptrdiff_t)(addr - jmp_rx) >> 2;
-
-    if (offset == sextreg(offset, 0, 26)) {
-        i1 = encode_sd10k16_insn(OPC_B, offset);
-        i2 = NOP;
-    } else {
-        tcg_debug_assert(offset == sextreg(offset, 0, 36));
-        lower = (int16_t)offset;
-        upper = (offset - lower) >> 16;
-
-        i1 = encode_dsj20_insn(OPC_PCADDU18I, TCG_REG_TMP0, upper);
-        i2 = encode_djsk16_insn(OPC_JIRL, TCG_REG_ZERO, TCG_REG_TMP0, lower);
-    }
-    uint64_t pair = ((uint64_t)i2 << 32) | i1;
-    qatomic_set((uint64_t *)jmp_rw, pair);
-    flush_idcache_range(jmp_rx, jmp_rw, 8);
-}
-
 /*
  * Entry-points
  */
@@ -1200,23 +1169,35 @@ static void tcg_out_exit_tb(TCGContext *s, uintptr_t a0)
 
 static void tcg_out_goto_tb(TCGContext *s, int which)
 {
-    /*
-     * Ensure that patch area is 8-byte aligned so that an
-     * atomic write can be used to patch the target address.
-     */
-    if ((uintptr_t)s->code_ptr & 7) {
-        tcg_out_nop(s);
-    }
+    /* Direct branch will be patched by tb_target_set_jmp_target. */
     set_jmp_insn_offset(s, which);
-    /*
-     * actual branch destination will be patched by
-     * tb_target_set_jmp_target later
-     */
-    tcg_out_opc_pcaddu18i(s, TCG_REG_TMP0, 0);
+    tcg_out_opc_b(s, 0);
+
+    /* When branch is out of range, fall through to indirect. */
+    tcg_out_ld(s, TCG_TYPE_PTR, TCG_REG_TMP0, TCG_REG_ZERO,
+               get_jmp_target_addr(s, which));
     tcg_out_opc_jirl(s, TCG_REG_ZERO, TCG_REG_TMP0, 0);
     set_jmp_reset_offset(s, which);
 }
 
+void tb_target_set_jmp_target(const TranslationBlock *tb, int n,
+                              uintptr_t jmp_rx, uintptr_t jmp_rw)
+{
+    uintptr_t addr = tb->jmp_target_addr[n];
+    ptrdiff_t offset = (ptrdiff_t)(addr - jmp_rx) >> 2;
+    tcg_insn_unit insn;
+
+    /* Either directly branch, or fall through to indirect branch. */
+    if (offset == sextreg(offset, 0, 26)) {
+        insn = encode_sd10k16_insn(OPC_B, offset);
+    } else {
+        /* LoongArch uses `andi zero, zero, 0` as NOP.  */
+        insn = OPC_ANDI;
+    }
+    qatomic_set((tcg_insn_unit *)jmp_rw, insn);
+    flush_idcache_range(jmp_rx, jmp_rw, 4);
+}
+
 static void tcg_out_op(TCGContext *s, TCGOpcode opc,
                        const TCGArg args[TCG_MAX_OP_ARGS],
                        const int const_args[TCG_MAX_OP_ARGS])
-- 
2.34.1