| Message ID | AANLkTingvVPJPfnaheLHWgpyGPuGbxGLXERsP+j0zjx5@mail.gmail.com |
|---|---|
| State | New |
| Headers | show |
On 08/01/2010 11:44 AM, Uros Bizjak wrote: > Hello! > > As discussed in the PR, stdarg pass depends on number of assignments > to ap.__offset location for correct operation. However, recent FRE/DCE > enhancements remove one assignment as a dead code, causing the test > failure: > > FAIL: gcc.c-torture/execute/stdarg-1.c execution, -O3 -fomit-frame-pointer > FAIL: gcc.c-torture/execute/stdarg-1.c execution, -O3 -g > > Attached patch marks __offset as volatile (suggested by Richi in > comment #39), preventing optimizations that could otherwise confuse > stdarg pass. > > 2010-08-01 Uros Bizjak <ubizjak@gmail.com> > > PR target/41089 > * config/alpha/alpha.c (alpha_build_builtin_va_list): Mark __offset > as volatile. > > Patch was bootstrapped and regression tested on > alphaev68-pc-linux-gnu, where it fixes the failure. Ug. That's awful. It means that when you really do have dead code it won't get deleted, and thus the stdarg pass won't be able to eliminate the stores that could have been prevented. I'm tempted to say just remove the entire optimization, but what's left after the volatile may be slightly better than no optimization at all -- i.e. we can determine that there were never any va_arg requests for double at all. So... patch ok. That said, I begin to think that a simpler solution in the long run might be to avoid expanding va_arg before pass_stdarg at all. Then we don't need to do any of this kind of queer code scanning or hacks in sra to prevent structure decomp. We'd simply scan for whether the data escapes, and with the help of a callback record the set of types read. After pass_stdarg, va_arg would be decomposed and we could perform all the usual cleanups. r~
Index: config/alpha/alpha.c =================================================================== --- config/alpha/alpha.c (revision 162794) +++ config/alpha/alpha.c (working copy) @@ -5948,6 +5948,7 @@ alpha_build_builtin_va_list (void) ofs = build_decl (BUILTINS_LOCATION, FIELD_DECL, get_identifier ("__offset"), integer_type_node); + TREE_THIS_VOLATILE (ofs) = 1; DECL_FIELD_CONTEXT (ofs) = record; DECL_CHAIN (ofs) = space;