On Mon, Mar 30, 2026 at 8:28 AM Richard Biener <[email protected]> wrote: > > > > > Am 30.03.2026 um 16:44 schrieb H.J. Lu <[email protected]>: > > > > On Mon, Mar 30, 2026 at 12:28 AM Richard Biener <[email protected]> wrote: > >> > >>> On Sat, 28 Mar 2026, H.J. Lu wrote: > >>> > >>> On Sat, Mar 28, 2026 at 1:16 AM Richard Biener <[email protected]> wrote: > >>>> > >>>> > >>>> > >>>>> Am 28.03.2026 um 03:36 schrieb H.J. Lu <[email protected]>: > >>>>> > >>>>> On Fri, Mar 27, 2026 at 4:14 AM Richard Biener <[email protected]> > >>>>> wrote: > >>>>>> > >>>>>> The following fixes a confusion seen in the x86 backend by > >>>>>> assign_parm_adjust_stack_rtl failing to trigger a local stack copy > >>>>>> for an incoming stack parameter that is not aligned according to > >>>>>> its type. The condition was introduced in r0-64961-gbfc45551d5ace4 > >>>>>> but there is the MEM_ALIGN (stack_parm) < PREFERRED_STACK_BOUNDARY > >>>>>> condition not triggering for the case in question where both > >>>>>> MEM_ALIGN and PREFERRED_STACK_BOUNDARY are 128. x86 supports > >>>>>> stack-realignment so we can honor the declared alignment and this > >>>>>> clears up the confusion. The following replaces the bound > >>>>>> by MAX_SUPPORTED_STACK_ALIGNMENT if SUPPORTS_STACK_ALIGNMENT > >>>>>> and the parameter has it's address taken and with BIGGEST_ALIGNMENT > >>>>>> if SUPPORTS_STACK_ALIGNMENT otherwise, only affecting x86 and nvptx > >>>>>> at this point. > >>>>>> > >>>>>> In addition to this it changes the i386 backends computation of > >>>>>> the maximum stack alignment used to bound itself to BIGGEST_ALIGNMENT > >>>>>> because the middle-end does not (and in general cannot) enforce actual > >>>>>> alignment of all stack objects according to their type. > >>>>>> > >>>>>> Bootstrapped and tested on x86_64-unknown-linux-gnu. > >>>>>> > >>>>>> Compared to v2 this uses BIGGEST_ALIGNMENT if the parameter does not > >>>>>> have its address taken and caps ix86_update_stack_alignment on > >>>>>> BIGGEST_ALIGNMENT as well. > >>>>>> > >>>>>> OK for the x86 parts? I assume the refined function.cc hunk is still > >>>>>> LGTM from Richard S. side. > >>>>>> > >>>>>> Note I still fail to see why we need to scan all insns in the backend > >>>>>> again after RTL expansion should have ensured to keep track of the > >>>>>> maximum needed stack alignment. But I'm trying to avoid touching > >>>>>> code I do not understand as much as possible. > >>>>>> > >>>>>> Thanks, > >>>>>> Richard. > >>>>>> > >>>>>> PR middle-end/120839 > >>>>>> * function.cc (assign_parm_adjust_stack_rtl): Get the parameter > >>>>>> as arugment. Adjust alignment check forcing a local copy for > >>>>>> SUPPORTS_STACK_ALIGNMENT targets if the argument is not aligned > >>>>>> to its type and the current alignment is less than > >>>>>> MAX_SUPPORTED_STACK_ALIGNMENT if the parameter has its address > >>>>>> taken or BIGGEST_ALIGNMENT otherwise. > >>>>>> (assign_parms): Adjust. > >>>>>> * config/i386/i386.cc (ix86_update_stack_alignment): Bound > >>>>>> recorded stack alignment requirement by BIGGEST_ALIGNMENT. > >>>>>> > >>>>>> * gcc.dg/torture/pr120839.c: New testcase. > >>>>>> * gcc.target/i386/pr120839-avx.c: Likewise. > >>>>>> --- > >>>>>> gcc/config/i386/i386.cc | 2 +- > >>>>>> gcc/function.cc | 14 +++++++++++--- > >>>>>> gcc/testsuite/gcc.dg/torture/pr120839.c | 7 +++++++ > >>>>>> gcc/testsuite/gcc.target/i386/pr120839-avx.c | 8 ++++++++ > >>>>>> 4 files changed, 27 insertions(+), 4 deletions(-) > >>>>>> create mode 100644 gcc/testsuite/gcc.dg/torture/pr120839.c > >>>>>> create mode 100644 gcc/testsuite/gcc.target/i386/pr120839-avx.c > >>>>>> > >>>>>> diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc > >>>>>> index 15e0dd547a9..f2a49bbaf46 100644 > >>>>>> --- a/gcc/config/i386/i386.cc > >>>>>> +++ b/gcc/config/i386/i386.cc > >>>>>> @@ -8627,7 +8627,7 @@ ix86_update_stack_alignment (rtx, const_rtx pat, > >>>>>> void *data) > >>>>>> unsigned int alignment = MEM_ALIGN (op); > >>>>>> > >>>>>> if (alignment > *p->stack_alignment) > >>>>>> - *p->stack_alignment = alignment; > >>>>>> + *p->stack_alignment = MIN (alignment, > >>>>>> BIGGEST_ALIGNMENT); > >>>>> > >>>>> This is wrong. X86 backend supports MAX_OFILE_ALIGNMENT > >>>>> stack alignment. > >>>>> > >>>>>> break; > >>>>>> } > >>>>>> else > >>>>>> diff --git a/gcc/function.cc b/gcc/function.cc > >>>>>> index bba05f3380d..41987b4f7a4 100644 > >>>>>> --- a/gcc/function.cc > >>>>>> +++ b/gcc/function.cc > >>>>>> @@ -2825,7 +2825,7 @@ assign_parm_remove_parallels (struct > >>>>>> assign_parm_data_one *data) > >>>>>> always valid and properly aligned. */ > >>>>>> > >>>>>> static void > >>>>>> -assign_parm_adjust_stack_rtl (struct assign_parm_data_one *data) > >>>>>> +assign_parm_adjust_stack_rtl (tree parm, struct assign_parm_data_one > >>>>>> *data) > >>>>>> { > >>>>>> rtx stack_parm = data->stack_parm; > >>>>>> > >>>>>> @@ -2840,7 +2840,15 @@ assign_parm_adjust_stack_rtl (struct > >>>>>> assign_parm_data_one *data) > >>>>>> MEM_ALIGN > >>>>>> (stack_parm)))) > >>>>>> || (data->nominal_type > >>>>>> && TYPE_ALIGN (data->nominal_type) > MEM_ALIGN (stack_parm) > >>>>>> - && MEM_ALIGN (stack_parm) < PREFERRED_STACK_BOUNDARY))) > >>>>>> + /* When we can re-align the stack ensure appropriate > >>>>>> alignment > >>>>>> + of the function local object up to BIGGEST_ALIGNMENT > >>>>>> if > >>>>>> + it is only accessed directly or up to the maximum > >>>>>> supported > >>>>>> + alignment if the address is exposed. */ > >>>>>> + && MEM_ALIGN (stack_parm) < (SUPPORTS_STACK_ALIGNMENT > >>>>>> + ? (TREE_ADDRESSABLE (parm) > >>>>>> + ? > >>>>>> MAX_SUPPORTED_STACK_ALIGNMENT > >>>>>> + : BIGGEST_ALIGNMENT) > >>>>>> + : > >>>>>> PREFERRED_STACK_BOUNDARY)))) > >>>>> > >>>>> I think it should be > >>>>> > >>>>> @@ -2840,7 +2840,11 @@ assign_parm_adjust_stack_rtl (struct > >>>>> assign_parm_data_one *data) > >>>>> MEM_ALIGN (stack_parm)))) > >>>>> || (data->nominal_type > >>>>> && TYPE_ALIGN (data->nominal_type) > MEM_ALIGN (stack_parm) > >>>>> - && MEM_ALIGN (stack_parm) < PREFERRED_STACK_BOUNDARY))) > >>>>> + /* The parm stack slot works if its address isn't taken. When > >>>>> + making a local copy, MAX_SUPPORTED_STACK_ALIGNMENT is > >>>>> + its maximum alignment. */ > >>>>> + && TREE_ADDRESSABLE (parm) > >>>>> + && MEM_ALIGN (stack_parm) < MAX_SUPPORTED_STACK_ALIGNMENT))) > >>>>> stack_parm = NULL; > >>>> > >>>> Even when not address taken the ABI guaranteed alignment of the stack > >>>> slot might be less than the declared alignment. If the alignment itself > >>>> is not exposed via the address we have still to avoid faulting, thus > >>>> BIGGEST_ALIGNMENT > >>> > >>> There should be no fault. Otherwise, caller will fault first when > >>> such an argument > >>> is pushed onto stack. > >> > >> I don't think this necessarily follows, the caller can end up using memcpy > >> while portions accessed in the callee can end up using aligned moves. > >> Also when the ABI effectively requires less alignment the caller has to > >> avoid using aligned accesses even when the formal alignment of the > >> argument is bigger. > >> > >> All this is about ABI mandated/guaranteed alignment being lower than > >> the types alignment. > > > > Shouldn't it be decided by backend? > > I’m not sure what there is to decide? The > Alignment as specified by the ABI is insufficient to guarantee the assignment > required for the type. That’s a fact. > > IMO we should diagnose such cases to the user, suggesting pass by reference > rather than by value and then honor the types alignment, doing ‚optimization > Where it does not matter‘ possibly in a target specific way - my approach > would have been to rely on BIGGEST_ALIGNMENT as that should cover all means > of access to an object - possibly further constrained by the objects natural > size (hoping alignment padding isn’t accessed). >
There are no functional issues when a misaligned argument is placed on stack. What benefits does it have to make an aligned copy when its address isn't taken? -- H.J.
