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Re: next glibc showstopper
- To: Ulrich Drepper <drepper at ipd dot info dot uni-karlsruhe dot de>
- Subject: Re: next glibc showstopper
- From: Bernd Schmidt <crux at hutch dot Informatik dot RWTH-Aachen dot DE>
- Date: Mon, 1 Dec 1997 14:20:23 +0100 (MET)
- cc: egcs at cygnus dot com, kenner at vlsi1 dot ultra dot nyu dot edu
> Here's the next problem which prevent successfully compiling the glibc
> with the current egcs on ix86:
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> double r (double, double);
>
> extern inline int
> bar (double x)
> {
> union { double d; int i[2]; } u = { d: x }; return u.i[1] < 0;
> }
>
> int
> foo (double d1, double d2)
> {
> double e = r (d1, d2);
>
> if (bar (d1) && bar (e)) <---- PROBLEM
> return 1;
> else
> return 5;
>
> return 0;
> }
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> The fault part of the code is this:
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 12: e8 fc ff ff ff call 13 <foo+0x13>
> 17: dd 45 08 fldl 0x8(%ebp)
> 1a: dd 5d f8 fstpl 0xfffffff8(%ebp)
>
> if (bar (d1) && bar (e))
> 1d: 83 7d fc 00 cmpl $0x0,0xfffffffc(%ebp)
> 21: 7d 0d jnl 30 <foo+0x30>
> 23: dd 5d f8 fstpl 0xfffffff8(%ebp) \ here is something
> 26: 7d 0a jnl 32 <foo+0x32> / missing
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> After the call to `r' (address 12) the return value is in %st(0). The
> according to the inline function `d1' is tested. It is copied on the
> stack (why?) and the correct word is tested against $0. This is ok.
>
> But now the return value must be checked. Here only the storing
> happens but NO compare instruction sets the flags. I.e., the flags as
> determined for `d1' are used.
That's because GCC thinks the flags are still valid from the previous
compare instruction. The NOTICE_UPDATE_CC macro fails to clear the remembered
status after the second store.
I've attached a patch which tries to solve this problem in final.c. I noticed
that there's a lot of code in i386.c which implements NOTICE_UPDATE_CC, but
I think the code which deals with invalidating cc_status could be moved to
a machine-independent file. The patch doesn't attempt to do this in the best
possible way, it only fixes the bug.
Note: there's another place in final.c where NOTICE_UPDATE_CC is used. I'm
not sure whether that place should be updated as well.
Bernd
*** ./final.c.orig-1 Sat Nov 29 20:46:49 1997
--- ./final.c Sat Nov 29 20:34:11 1997
*************** final (first, file, optimize, prescan)
*** 1332,1337 ****
--- 1332,1351 ----
add_bb (file);
}
+ #ifdef HAVE_cc0
+ /* Invalidate cc_status if remembered values are clobbered by an
+ insn. Called via note_stores. */
+ static void
+ invalidate_cc_status (dest, set)
+ rtx set;
+ rtx dest;
+ {
+ if ((cc_status.value1 && reg_overlap_mentioned_p (dest, cc_status.value1))
+ || (cc_status.value2 && reg_overlap_mentioned_p (dest, cc_status.value2)))
+ CC_STATUS_INIT;
+ }
+ #endif
+
/* The final scan for one insn, INSN.
Args are same as in `final', except that INSN
is the insn being scanned.
*************** final_scan_insn (insn, file, optimize, p
*** 2122,2131 ****
--- 2136,2148 ----
cc_prev_status = cc_status;
/* Update `cc_status' for this instruction.
+ First, forget values that this insn clobbers.
+
The instruction's output routine may change it further.
If the output routine for a jump insn needs to depend
on the cc status, it should look at cc_prev_status. */
+ note_stores (PATTERN (insn), invalidate_cc_status);
NOTICE_UPDATE_CC (body, insn);
#endif