[PR middle-end/56108] handle transactions with ASMs in the first block

[Aldy Hernandez <aldyh at redhat dot com>]

In the following test, the first statement of a relaxed transaction is 
an inline asm:

  __transaction_relaxed { __asm__(""); }

Since we bypass inserting BUILT_IN_TM_IRREVOCABLE at the beginning of 
transactions that are sure to be irrevocable, later when we try to 
expand the transaction, we ICE when we encounter the inline asm.

Currently, we bypass the TM_IRREVOCABLE call here:

 for (region = d->all_tm_regions; region; region = region->next)
    {
      /* If we're sure to go irrevocable, don't transform anything.  */
      if (d->irrevocable_blocks_normal
      && bitmap_bit_p (d->irrevocable_blocks_normal,
               region->entry_block->index))
    {
      transaction_subcode_ior (region, GTMA_DOES_GO_IRREVOCABLE);
      transaction_subcode_ior (region, GTMA_MAY_ENTER_IRREVOCABLE);
      continue;
    }

If I understand this correctly, ideally a transaction marked as 
doesGoIrrevocable shouldn't bother instrumenting the statements inside, 
since the runtime will go irrevocable immediately.  In which case, we 
can elide the instrumentation altogether as the attached patch does.

If my analysis is correct, then testsuite/gcc.dg/tm/memopt-1.c would 
surely go irrevocable, thus requiring no instrumentation, causing the 
memory optimizations to get skipped altogether.  In which case, it's 
best to mark the function calls as safe, so they don't cause the 
transaction to become obviously irrevocable.

Is this correct?  If so, OK?

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