[LLVMdev] [RFC/PATCH][4/4] Support System Z as host architecture

[Ulrich Weigand]

Hello,

this last patch is strictly-speaking not related to System Z as target
platform, but to running LLVM on (Linux on) System Z as a *host*
architecture.  Most of the LLVM and Clang code base builds and works fine
on System Z, however, there is one problem relating to use of signal
handlers to react to crashes (SIGSEGV, SIGILL etc.).

The llvm::sys::AddSignalHandler (and related) function in
lib/Support/Unix/Signals.inc currently registers a signal handler routine
via "sigaction".  When this handler is called due to a SIGSEGV, SIGILL or
similar signal, it will show a stack backtrace, deactivate the handler, and
then simply return to the operating system.  The intent is that the OS will
now retry execution at the same location as before, which ought to again
trigger the same error condition and cause the same signal to be delivered
again.  Since the hander is now deactivated, the OS will take its default
action (usually, terminate the program and possibly create a core dump).

However, this method doesn't work reliably on System Z:  With certain
signals (namely SIGILL, SIGFPE, and SIGTRAP), the program counter stored by
the kernel on the signal stack frame (which is the location where execution
will resume) is not the instruction that triggered the fault, but then
instruction *after it*.  When the LLVM signal handler simply returns to the
kernel, execution will then resume at *that* address, which will not
trigger the problem again, but simply go on and execute potentially
unrelated code leading to random errors afterwards.

To fix this, the patch simply goes and re-raises the signal in question
directly from the handler instead of returning from it.  This is done only
on System Z and only for those signals that have this particular problem.
[ This fixes e.g. the otherwise failing Driver/output-file-cleanup.c Clang
test case. ]

[ I guess this patch is really independent of the System Z backend; it
ought to go in in any case to fix cross-compilers hosted on System Z ... ]

[See also LLVMdev for the full set of SystemZ patches and discussion.]

Bye,
Ulrich

(See attached file: diff-llvm-systemzsig)
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