[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

[Peter Maydell]

Unfortunately the fix is too big for this point in the 4.0 release
cycle; it'll go into 4.1.

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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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