On Tue, 1 Mar 2016, Ramana Radhakrishnan wrote: > > > On 01/03/16 09:54, Richard Biener wrote: > > On Tue, 1 Mar 2016, James Greenhalgh wrote: > > > >> On Tue, Mar 01, 2016 at 10:21:27AM +0100, Richard Biener wrote: > >>> On Mon, 29 Feb 2016, James Greenhalgh wrote: > >>> > >>>> On Fri, Feb 26, 2016 at 09:32:53AM +0100, Richard Biener wrote: > >>>>> > >>>>> The following fixes PR69951, hopefully the last case of decl alias > >>>>> issues with alias analysis. This time it's points-to and the DECL_UIDs > >>>>> used in points-to sets not being canonicalized. > >>>>> > >>>>> The simplest (and cheapest) fix is to make aliases refer to the > >>>>> ultimate alias target via their DECL_PT_UID which we conveniently > >>>>> have available. > >>>>> > >>>>> Bootstrapped and tested on x86_64-unknown-linux-gnu, applied to trunk. > >>>>> > >>>>> Richard. > >>>>> > >>>>> 2016-02-26 Richard Biener <rguent...@suse.de> > >>>>> > >>>>> PR tree-optimization/69551 > >>>>> * tree-ssa-structalias.c (get_constraint_for_ssa_var): When > >>>>> looking through aliases adjust DECL_PT_UID to refer to the > >>>>> ultimate alias target. > >>>>> > >>>>> * gcc.dg/torture/pr69951.c: New testcase. > >>>> > >>>> I see this new testcase failing on an ARM target as so: > >>>> > >>>> /tmp/ccChjoFc.s: Assembler messages: > >>>> /tmp/ccChjoFc.s:21: Warning: [-mwarn-syms]: Assignment makes a > >>>> symbol match an ARM instruction: b > >>>> > >>>> FAIL: gcc.dg/torture/pr69951.c -O0 (test for excess errors) > >>>> > >>>> But I haven't managed to reproduce it outside of the test environment. > >>>> > >>>> The fix looks trivial, rename b to anything else you fancy (well... stay > >>>> clear of add and ldr). I'll put a fix in myself if I can manage to get > >>>> this to reproduce - though if anyone else wants to do it I won't be > >>>> offended :-). > >>> > >>> Huh, I wonder what's the use of such warning. After all 'ldr' is a valid > >>> C symbol name, too. In fact my cross arm as doesn't report this > >>> warning (binutils 2.25.0) > >>> > >>>> arm-suse-linux-gnueabi-as t.s -mwarn-syms > >>> Assembler messages: > >>> Error: unrecognized option -mwarn-syms > >> > >> Right, I've figured out the set of conditions... You need to be targeting > >> an arm-*-linux-* system to make sure that the ASM_OUTPUT_DEF definition > >> from config/arm/linux-elf.h is pulled in. This causes us to emit: > >> > >> b = a > >> > >> Rather than > >> > >> .set b,a > >> > >> Writing it as "b = a" causes the warning added to resolve binutils > >> PR18347 [1] to kick in, so you need binutils > 2.26 or to have backported > >> that patch). > >> > >> Resolving it by hacking the testcase would be one fix, but I wonder why the > >> ARM port prefers to emit "b = a" in a linux environment if .set does the > >> same thing and always avoids the warning? Maybe Ramana/Richard/Kyrill/Nick > >> remember? > >> (AArch64 does the same thing, but the AArch64 gas port doesn't > >> have the PR18347 fix). > > > > So does b = a define a macro then and the warning is to avoid you > > doing > > > > > I don't think this is a macro, b = a seems to be a way of setting the > value of a to b. in the assembler. If a is an expression , then I > believe the expression is resolved at assemble time - (b ends up being a > symbol in the symbol table produced with the value of a) in this case > the address of a. .set b, a achieves the same thing from my experiments > and reading of the sources. The reason ports appear to choose not to use > the .set a, b idiom is if the assembler syntax has hijacked the .set > directive for something else. Thus I don't see why we use the > ASM_OUTPUT_DEF form in the GNU/Linux port TBH rather than the .set form > especially as we don't reuse .set for anything else in the ARM assembler > port and SET_ASM_OP is defined in config/arm/aout.h. > > The use of .set in the arm port of glibc for assembler code for the same > purpose seems to also vindicate that kind of thought. > No reasons were given here[1], maybe Nick or Richard remember from > nearly 18 years ago ;) > > > Therefore this seems to be an assembler bug to me in that it doesn't > allow such an assignment of values, and a backend wart to me that we > have ASM_OUTPUT_DEF defined for no good reason. So, a patch that removes > ASM_OUTPUT_DEF from linux-elf.h seems obvious to me pending testing. > > > Nick , Richard - any thoughts ?
So - why does it warn at all for this? And why does it only warn for b = a and not .set b, a? IMHO the warning is simply bogus? Richard. > > regards > Ramana > > 1. https://gcc.gnu.org/ml/gcc-patches/1998-10/msg00701.html > > > > > ... > > > > ldr 0, 1 (or whatever correct ldr instruction) > > > > and have that ldr replaced by b? > > > > Then it's a bug to emit aliases in this form and I hope .set ldr, b > > doesn't have the same effect. > > > > Richard. > > > > -- Richard Biener <rguent...@suse.de> SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)