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Re: [PATCH] Fix DCE REG_LIBCALL note moving from noop move insns (PR rtl-optimization/33644)

On Fri, Oct 19, 2007 at 03:04:49PM +0200, Steven Bosscher wrote:
> > > Compile with -fpic.  There should only be one call to __tls_get_addr
> > > to get a base address.  Without libcalls there will be two calls.  The
> > > base address is not exposed in tree-ssa.
> >
> > But isn't the function marked const?  (Is that the right flag?)
> __tls_get_addr is not const, and not pure either.

gcc actually marks them as CONST_OR_PURE_CALL_P (which is a little bit of
lie, as glibc needs to write some memory as part of resolving __tls_get_addr
for the first time, but a) nothing actually cares if it is not called at all
b) nothing cares if it is called just once or several times).

Just tried
static __thread int a, b;

int foo (void)
  return a + b;

void bar (int x, int y)
  a = x;
  b = y;

with -O2 -m64 -fpic on x86_64 and -O2 -m{32,64} -fpic on ppc64 and while
4.1/4.2 have just one __tls_get_addr call in each fn, the trunk has 2 in each.
CSE does its job, replaces the pseudo set by second REG_RETVAL insn
by the pseudo set by first REG_RETVAL, but as dce.c doesn't handle libcalls
properly, nothing ever purges the dead libcall.


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