Filed PR jit/66811: jit jumps aren't compilable as C (Re: A possible code generation issue)

[David Malcolm]

On Mon, 2015-07-06 at 22:29 +0100, Dibyendu Majumdar wrote:
> On 4 July 2015 at 23:09, Dibyendu Majumdar <mobile@majumdar.org.uk> wrote:
> > On 4 July 2015 at 22:41, David Malcolm <dmalcolm@redhat.com> wrote:
> >>> Adding the -fno-strict-aliasing appears to resolve the issue with -O2
> >>> and -O3 but with this enabled the benchmarks are degraded.
> >>
> >> If "-fno-strict-aliasing" resolved this, then that suggests that there
> >> are various casts in the code that if this were C would take us into
> >> "undefined behavior" territory, and the optimizer is trying a little too
> >> hard.   This isn't C, but similar rules apply.  If that's what this is,
> >> then this is at least a documentation issue with libgccjit; we need to
> >> document what the rules are.
> >>
> >>
> >> Of course, it could well be a bug at my end.
> >>
> 
> Dave,
> 
> Attached is a hacked version in C of the dump from the code. I thought
> it might be useful to generate code directly using gcc -O3 -S on this
> file - from what I can tell the generated code looks different from
> what I am getting in libgccjit.
> 
> BTW the dump of types is problematic in many ways:
> 1) types are in wrong order
> 2) function types are incorrectly output
> 3) array types are incorrectly output
> 4) in one case as least a pointer was missing.
> 
> It would be useful if the generated dump was correct C so that it can
> directly compiled in gcc.

Agreed.  I've filed this as an RFE for the jit as:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66811