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[Bug 1912065] Re: Segfaults in tcg/optimize.c:212 after commit 7c7972160

From: Richard Henderson
Subject: [Bug 1912065] Re: Segfaults in tcg/optimize.c:212 after commit 7c79721606be11b5bc556449e5bcbc331ef6867d
Date: Mon, 18 Jan 2021 17:27:21 -0000

The problem is that we're now generating many more temporaries
than before, and running out of the statically allocated amount.
Changing a debug assert to a full assert will change the SEGV
into an ABRT.  :-)

diff --git a/tcg/tcg.c b/tcg/tcg.c
index 8f8badb61c..c376afe56a 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -1207,7 +1207,7 @@ void tcg_func_start(TCGContext *s)
 static inline TCGTemp *tcg_temp_alloc(TCGContext *s)
     int n = s->nb_temps++;
-    tcg_debug_assert(n < TCG_MAX_TEMPS);
+    g_assert(n < TCG_MAX_TEMPS);
     return memset(&s->temps[n], 0, sizeof(TCGTemp));

The problem can be worked around temporarily by increasing the
number of temporaries:

diff --git a/include/tcg/tcg.h b/include/tcg/tcg.h
index 504c5e9bb0..8fe32bb03c 100644
--- a/include/tcg/tcg.h
+++ b/include/tcg/tcg.h
@@ -275,7 +275,7 @@ typedef struct TCGPool {
 #define TCG_POOL_CHUNK_SIZE 32768
-#define TCG_MAX_TEMPS 512
+#define TCG_MAX_TEMPS 2048
 #define TCG_MAX_INSNS 512
 /* when the size of the arguments of a called function is smaller than

But a proper solution is to dynamically allocate the temps.

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  Segfaults in tcg/optimize.c:212 after commit

Status in QEMU:
  In Progress

Bug description:
  QEMU segfaults to NULL dereference in tcg/optimize.c:212 semi-randomly
  after commit 7c79721606be11b5bc556449e5bcbc331ef6867d

  Exception Type:        EXC_BAD_ACCESS (SIGSEGV)
  Exception Codes:       KERN_INVALID_ADDRESS at 0x0000000000000020
  Exception Note:        EXC_CORPSE_NOTIFY


  Thread 4 Crashed:
  0   qemu-system-ppc                   0x0000000109cd26d2 tcg_opt_gen_mov + 
178 (optimize.c:212)
  1   qemu-system-ppc                   0x0000000109ccf838 tcg_optimize + 5656
  2   qemu-system-ppc                   0x0000000109c27600 tcg_gen_code + 64 
  3   qemu-system-ppc                   0x0000000109c17b6d tb_gen_code + 493 
  4   qemu-system-ppc                   0x0000000109c16085 tb_find + 41 
(cpu-exec.c:454) [inlined]
  5   qemu-system-ppc                   0x0000000109c16085 cpu_exec + 2117 
  6   qemu-system-ppc                   0x0000000109c09ac3 tcg_cpus_exec + 35 
  7   qemu-system-ppc                   0x0000000109c75edd rr_cpu_thread_fn + 
445 (tcg-cpus-rr.c:217)
  8   qemu-system-ppc                   0x0000000109e41fae qemu_thread_start + 
126 (qemu-thread-posix.c:521)
  9   libsystem_pthread.dylib           0x00007fff2038e950 _pthread_start + 224
  10  libsystem_pthread.dylib           0x00007fff2038a47b thread_start + 15

  Here the crash is in tcg/optimize.c line 212:

    mask = si->mask;

  "si" is NULL. The NULL value arises from tcg/optimize.c line 198:

   si = ts_info(src_ts);

  I did not attempt to determine the root cause of this issue, however.
  It clearly is related to the "tcg/optimize" changes in this commit.
  The previous commit c0dd6654f207810b16a75b673258f5ce2ceffbf0 doesn't

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