[Qemu-devel] [Bug 1824853] Re: 4.0.0-rc3 crashes with tcg/tcg.c:3952: tcg_gen_code: Assertion `s->gen_insn_end_off[num_insns] == off' failed

[Richard Henderson]

Returning -1 does not help because all that signals that the buffer is full.
We then flush the buffer and try again, assuming the at the buffer will not 
fill.
Given that the buffer is usually many megabytes, this is reasonable.

We need something different to signal that the buffer is not full, but that
another offset has overflowed.

** Changed in: qemu
       Status: New => Confirmed

** Changed in: qemu
       Status: Confirmed => In Progress

** Changed in: qemu
     Assignee: (unassigned) => Richard Henderson (rth)

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Title:
  4.0.0-rc3 crashes with tcg/tcg.c:3952: tcg_gen_code: Assertion
  `s->gen_insn_end_off[num_insns] == off' failed

Status in QEMU:
  In Progress

Bug description:
  I tried to bootstrap and regtested gcc trunk (gcc svn rev 270278,
  datestamp 20190411) inside my arm64-gentoo installation under qemu-
  system-aarch64.

  Qemu version was 4.0.0-rc3 and -cpu cortex-a57. Qemu configured with
  only --target-list=aarch64-softmmu,aarch64-linux-user and compiled
  using gcc "version 5.5.0 20171010 (Ubuntu 5.5.0-12ubuntu1~16.04)".

  Executable created from gcc/testsuite/gcc.target/aarch64/advsimd-
  intrinsics/vldX.c compiled with -O2 crashed the whole qemu-system.

  To investigate a bit I also manually run
  ~/gcc/inst/trunk/bin/gcc 
~/gcc/src/trunk/gcc/testsuite/gcc.target/aarch64/advsimd-intrinsics/vldX.c
  with different options like:
  -O0 -lm -o d0.exe
  -O1 -lm -o d1.exe
  -O2 -lm -o d2.exe
  -O0 -static -lm -o s0.exe
  -O1 -static -lm -o s1.exe
  -O2 -static -lm -o s2.exe

  So, now I have 6 different arm64 executables created with different 
optimization levels. O0 and O1 versions run ok.
  Three sN.exe static executables I've also tried in qemu user mode (with same 
-cpu), no issue in user mode.

  And inside qemu-system I can see that
  running "d2.exe" (attached) gives:
  tcg/tcg.c:3952: tcg_gen_code: Assertion `s->gen_insn_end_off[num_insns] == 
off' failed.

  And running "s2.exe" gives:
  tcg/tcg.c:320: set_jmp_reset_offset: Assertion `s->tb_jmp_reset_offset[which] 
== off' failed.

  It seems like this test is an counter-example for logic that
  "tcg_ctx->nb_ops < 4000" implies tcg will fit into 16-bit signed size
  (see tcg_op_buf_full comments).

  Richard's changes in abebf92597186 and 9f754620651d were not enough, 
translation block must be smaller, or we have to find some proper way to bail 
out when buffer overflows.
  I don't know why this situation is not caught by code_gen_highwater logic in 
tcg.c

  I've also tried this "bail out" patch

  diff --git a/tcg/tcg.c b/tcg/tcg.c
  --- a/tcg/tcg.c
  +++ b/tcg/tcg.c
  @@ -3949,7 +3949,8 @@ int tcg_gen_code(TCGContext *s, TranslationBlock *tb)
                   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);
  +                if (s->gen_insn_end_off[num_insns] != off)
  +                  return -1;
               }
               num_insns++;
               for (i = 0; i < TARGET_INSN_START_WORDS; ++i) {

  But then running "d2.exe" just hangs the whole qemu-system. It seems
  that when tcg_gen_code return -1 (like in highwater logic mentioned
  before), we just re-call it again and again.

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