tcg: Reduce max TB opcode count

Also, assert that we don't overflow any of two different offsets into
the TB. Both unwind and goto_tb both record a uint16_t for later use.

This fixes an arm-softmmu test case utilizing NEON in which there is
a TB generated that runs to 7800 opcodes, and compiles to 96k on an
x86_64 host.  This overflows the 16-bit offset in which we record the
goto_tb reset offset.  Because of that overflow, we install a jump
destination that goes to neverland.  Boom.

With this reduced op count, the same TB compiles to about 48k for
aarch64, ppc64le, and x86_64 hosts, and neither assertion fires.

Cc: qemu-stable@nongnu.org
Reported-by: "Jason A. Donenfeld" <Jason@zx2c4.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

10 files changed, 26 insertions, 13 deletions

diff --git a/tcg/aarch64/tcg-target.inc.c b/tcg/aarch64/tcg-target.inc.c
index be3192078d..4562d36d1b 100644
--- a/tcg/aarch64/tcg-target.inc.c
+++ b/tcg/aarch64/tcg-target.inc.c
@@ -1733,7 +1733,7 @@ static void tcg_out_op(TCGContext *s, TCGOpcode opc,
             tcg_out_insn(s, 3305, LDR, offset, TCG_REG_TMP);
         }
         tcg_out_insn(s, 3207, BR, TCG_REG_TMP);
-        s->tb_jmp_reset_offset[a0] = tcg_current_code_size(s);
+        set_jmp_reset_offset(s, a0);
         break;
 
     case INDEX_op_goto_ptr:
diff --git a/tcg/arm/tcg-target.inc.c b/tcg/arm/tcg-target.inc.c
index 56a32a470f..e1fbf465cb 100644
--- a/tcg/arm/tcg-target.inc.c
+++ b/tcg/arm/tcg-target.inc.c
@@ -1822,7 +1822,7 @@ static inline void tcg_out_op(TCGContext *s, TCGOpcode opc,
                 tcg_out_movi32(s, COND_AL, base, ptr - dil);
             }
             tcg_out_ld32_12(s, COND_AL, TCG_REG_PC, base, dil);
-            s->tb_jmp_reset_offset[args[0]] = tcg_current_code_size(s);
+            set_jmp_reset_offset(s, args[0]);
         }
         break;
     case INDEX_op_goto_ptr:
diff --git a/tcg/i386/tcg-target.inc.c b/tcg/i386/tcg-target.inc.c
index 0d0ff524b7..e87b0d445e 100644
--- a/tcg/i386/tcg-target.inc.c
+++ b/tcg/i386/tcg-target.inc.c
@@ -2245,7 +2245,7 @@ static inline void tcg_out_op(TCGContext *s, TCGOpcode opc,
             tcg_out_modrm_offset(s, OPC_GRP5, EXT5_JMPN_Ev, -1,
                                  (intptr_t)(s->tb_jmp_target_addr + a0));
         }
-        s->tb_jmp_reset_offset[a0] = tcg_current_code_size(s);
+        set_jmp_reset_offset(s, a0);
         break;
     case INDEX_op_goto_ptr:
         /* jmp to the given host address (could be epilogue) */
diff --git a/tcg/mips/tcg-target.inc.c b/tcg/mips/tcg-target.inc.c
index ca5f1d4894..cff525373b 100644
--- a/tcg/mips/tcg-target.inc.c
+++ b/tcg/mips/tcg-target.inc.c
@@ -1744,7 +1744,7 @@ static inline void tcg_out_op(TCGContext *s, TCGOpcode opc,
             tcg_out_opc_reg(s, OPC_JR, 0, TCG_TMP0, 0);
         }
         tcg_out_nop(s);
-        s->tb_jmp_reset_offset[a0] = tcg_current_code_size(s);
+        set_jmp_reset_offset(s, a0);
         break;
     case INDEX_op_goto_ptr:
         /* jmp to the given host address (could be epilogue) */
diff --git a/tcg/ppc/tcg-target.inc.c b/tcg/ppc/tcg-target.inc.c
index 86f7de5f7e..c2f729ee8f 100644
--- a/tcg/ppc/tcg-target.inc.c
+++ b/tcg/ppc/tcg-target.inc.c
@@ -2025,10 +2025,10 @@ static void tcg_out_op(TCGContext *s, TCGOpcode opc, const TCGArg *args,
         }
         tcg_out32(s, MTSPR | RS(TCG_REG_TB) | CTR);
         tcg_out32(s, BCCTR | BO_ALWAYS);
-        s->tb_jmp_reset_offset[args[0]] = c = tcg_current_code_size(s);
+        set_jmp_reset_offset(s, args[0]);
         if (USE_REG_TB) {
             /* For the unlinked case, need to reset TCG_REG_TB.  */
-            c = -c;
+            c = -tcg_current_code_size(s);
             assert(c == (int16_t)c);
             tcg_out32(s, ADDI | TAI(TCG_REG_TB, TCG_REG_TB, c));
         }
diff --git a/tcg/s390/tcg-target.inc.c b/tcg/s390/tcg-target.inc.c
index 9af6dcef05..17c435ade5 100644
--- a/tcg/s390/tcg-target.inc.c
+++ b/tcg/s390/tcg-target.inc.c
@@ -1783,7 +1783,7 @@ static inline void tcg_out_op(TCGContext *s, TCGOpcode opc,
             /* and go there */
             tcg_out_insn(s, RR, BCR, S390_CC_ALWAYS, TCG_REG_TB);
         }
-        s->tb_jmp_reset_offset[a0] = tcg_current_code_size(s);
+        set_jmp_reset_offset(s, a0);
 
         /* For the unlinked path of goto_tb, we need to reset
            TCG_REG_TB to the beginning of this TB.  */
diff --git a/tcg/sparc/tcg-target.inc.c b/tcg/sparc/tcg-target.inc.c
index bc673bd8c6..04bdc3df5e 100644
--- a/tcg/sparc/tcg-target.inc.c
+++ b/tcg/sparc/tcg-target.inc.c
@@ -1388,12 +1388,12 @@ static void tcg_out_op(TCGContext *s, TCGOpcode opc,
             tcg_out_arithi(s, TCG_REG_G0, TCG_REG_TB, 0, JMPL);
             tcg_out_nop(s);
         }
-        s->tb_jmp_reset_offset[a0] = c = tcg_current_code_size(s);
+        set_jmp_reset_offset(s, a0);
 
         /* For the unlinked path of goto_tb, we need to reset
            TCG_REG_TB to the beginning of this TB.  */
         if (USE_REG_TB) {
-            c = -c;
+            c = -tcg_current_code_size(s);
             if (check_fit_i32(c, 13)) {
                 tcg_out_arithi(s, TCG_REG_TB, TCG_REG_TB, c, ARITH_ADD);
             } else {
diff --git a/tcg/tcg.c b/tcg/tcg.c
index 1d1dfd7f7c..f27b22bd3c 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -306,6 +306,14 @@ TCGLabel *gen_new_label(void)
     return l;
 }
 
+static void set_jmp_reset_offset(TCGContext *s, int which)
+{
+    size_t off = tcg_current_code_size(s);
+    s->tb_jmp_reset_offset[which] = off;
+    /* Make sure that we didn't overflow the stored offset.  */
+    assert(s->tb_jmp_reset_offset[which] == off);
+}
+
 #include "tcg-target.inc.c"
 
 /* compare a pointer @ptr and a tb_tc @s */
@@ -3532,7 +3540,10 @@ int tcg_gen_code(TCGContext *s, TranslationBlock *tb)
             break;
         case INDEX_op_insn_start:
             if (num_insns >= 0) {
-                s->gen_insn_end_off[num_insns] = tcg_current_code_size(s);
+                size_t off = tcg_current_code_size(s);
+                s->gen_insn_end_off[num_insns] = off;
+                /* Assert that we do not overflow our stored offset.  */
+                assert(s->gen_insn_end_off[num_insns] == off);
             }
             num_insns++;
             for (i = 0; i < TARGET_INSN_START_WORDS; ++i) {
diff --git a/tcg/tcg.h b/tcg/tcg.h
index 532d2a0710..f9f12378e9 100644
--- a/tcg/tcg.h
+++ b/tcg/tcg.h
@@ -850,9 +850,11 @@ static inline bool tcg_op_buf_full(void)
     /* This is not a hard limit, it merely stops translation when
      * we have produced "enough" opcodes.  We want to limit TB size
      * such that a RISC host can reasonably use a 16-bit signed
-     * branch within the TB.
+     * branch within the TB.  We also need to be mindful of the
+     * 16-bit unsigned offsets, TranslationBlock.jmp_reset_offset[]
+     * and TCGContext.gen_insn_end_off[].
      */
-    return tcg_ctx->nb_ops >= 8000;
+    return tcg_ctx->nb_ops >= 4000;
 }
 
 /* pool based memory allocation */
diff --git a/tcg/tci/tcg-target.inc.c b/tcg/tci/tcg-target.inc.c
index cc949bea85..62ed097254 100644
--- a/tcg/tci/tcg-target.inc.c
+++ b/tcg/tci/tcg-target.inc.c
@@ -574,7 +574,7 @@ static void tcg_out_op(TCGContext *s, TCGOpcode opc, const TCGArg *args,
             /* Indirect jump method. */
             TODO();
         }
-        s->tb_jmp_reset_offset[args[0]] = tcg_current_code_size(s);
+        set_jmp_reset_offset(s, args[0]);
         break;
     case INDEX_op_br:
         tci_out_label(s, arg_label(args[0]));