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 ? 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. >