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Re: [PATCH PR62151]Fix uninitialized register issue caused by distribute_notes in combine pass
- From: Segher Boessenkool <segher at kernel dot crashing dot org>
- To: Ulrich Weigand <uweigand at de dot ibm dot com>
- Cc: Jeff Law <law at redhat dot com>, "Bin.Cheng" <amker dot cheng at gmail dot com>, Richard Earnshaw <rearnsha at arm dot com>, Bin Cheng <bin dot cheng at arm dot com>, "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>
- Date: Tue, 2 Sep 2014 08:40:50 -0500
- Subject: Re: [PATCH PR62151]Fix uninitialized register issue caused by distribute_notes in combine pass
- Authentication-results: sourceware.org; auth=none
- References: <20140901175036 dot GA6643 at gate dot crashing dot org> <201409021210 dot s82CAWsP031216 at d06av02 dot portsmouth dot uk dot ibm dot com>
On Tue, Sep 02, 2014 at 02:10:32PM +0200, Ulrich Weigand wrote:
> In any case, this test in can_combine_p rejects a combination for *two*
> different issues. One is the earlyclobber problem, which is what that
> 2004 thread was about, and which my patch back then relaxed for fixed
> hard register.
>
> However, this doesn't seem to apply to the example above; that is really
> about the second problem: don't substitute into a clobber.
Right.
> I understand the reason why this particular substitution is rejected is
> simply that if it weren't, we'd be substituting flags:CC=cmp(r84:SI,0x1)
> into clobber flags:CC, resulting in clobber cmp(r84:SI,0x1), which is
> invalid RTL.
I checked, and that is indeed what combine does. How silly.
> Now I guess this check could be relaxed if somewhere else in combine we'd
> recognize the substitution into a clobber and simply omit it in that case.
Yeah.
In the testcase, combine tries combining 76,77 (77 is that clobbering
insn) and refuses it; then it tries 32,76,77 and refuses it; and then
it tries 32,76,77,43 and allows it (it doesn't do this check at all,
77 is not i3, combine omits the clobber completely). Which is inconsistent.
What a mess. Thanks for looking!
Segher