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Re: [PATCH] target/68972 - g++.dg/cpp1y/vla-initlist1.C test case fails on powerpc64le

From: Jason Merrill <jason at redhat dot com>
To: Martin Sebor <msebor at gmail dot com>, Gcc Patch List <gcc-patches at gcc dot gnu dot org>
Cc: seurer at linux dot vnet dot ibm dot com
Date: Tue, 2 Feb 2016 11:18:56 +0100
Subject: Re: [PATCH] target/68972 - g++.dg/cpp1y/vla-initlist1.C test case fails on powerpc64le
Authentication-results: sourceware.org; auth=none
References: <56AA70A1 dot 8030303 at gmail dot com> <56AA9BF1 dot 4070100 at redhat dot com> <56AABF96 dot 7010101 at gmail dot com>

On 01/29/2016 02:25 AM, Martin Sebor wrote:

What seems to happen is that the call to __builtin_alloca_with_align
uses the stdu (store with update) instruction to store and bump down
the stack pointer (SP) at the same time (standard for powerpc63le)
to make room for the VLA.  The subsequent code then reads the saved
value of the SP and uses it as the address of the VLA. Since the SP
is 64 bits wide, it clobbers the first two words of the VLA.  The
test looks at the second element, expecting it to be unchanged, but
what it finds is the upper word of the saved SP. Since the saved SP
value doesn't get read I don't see anything wrong with this.

Interesting. What's the point of storing the SP if it isn't going to beused to restore it later?

I think I'd prefer to disable the test on targets with quirks like thisrather than everywhere.


Jason

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