On 01/03/16 10:49, Richard Biener wrote: > 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? >
Because b is the mnemonic for a branch instruction. What follows is probably garbage in reality if you start from that point. > IMHO the warning is simply bogus? I think the grammar for the '=' assignment is at best dubious, given that the LHS could be any label and there's no guarantee that won't conflict with a mnemonic. .set is clearly superior in that respect. R. > > 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. >>> >> >> >
