Hi!
Richard, as the original author of 'SSA_NAME_POINTS_TO_READONLY_MEMORY':
2018 commit 6214d5c7e7470bdd5ecbeae668c2522551bfebbc (Subversion r263958)
"Move const_parm trick to generic code"; 'gcc/tree.h':
/* Nonzero if this SSA_NAME is known to point to memory that may not
be written to. This is set for default defs of function parameters
that have a corresponding r or R specification in the functions
fn spec attribute. This is used by alias analysis. */
#define SSA_NAME_POINTS_TO_READONLY_MEMORY(NODE) \
SSA_NAME_CHECK (NODE)->base.deprecated_flag
..., may I ask you to please help review the following patch
(full-quoted)?
For context: this patch here ("second patch") depends on a first patch:
"[PATCH, OpenACC 2.7] readonly modifier support in front-ends". That one
is still under review/rework; so you're not able to apply this second
patch here.
In a nutshell: a 'readonly' modifier has been added to the OpenACC
'copyin' clause (copy host to device memory, don't copy back at end of
region):
| If the optional 'readonly' modifier appears, then the implementation may
assume that the data
| referenced by _var-list_ is never written to within the applicable region.
That is, for example (untested):
#pragma acc routine
void escape(int *);
int x[32] = [...];
#pragma acc parallel copyin(readonly: x)
{
int a1 = x[3];
escape(x);
int a2 = x[3]; // Per 'readonly', don't need to reload 'x[3]' here.
//x[22] = 0; // Invalid -- but no diagnostic mandated.
}
What Chung-Lin's first patch does is mark the OMP clause for 'x' (not the
'x' decl itself!) as 'readonly', via a new 'OMP_CLAUSE_MAP_READONLY'
flag.
The actual optimization then is done in this second patch. Chung-Lin
found that he could use 'SSA_NAME_POINTS_TO_READONLY_MEMORY' for that.
I don't have much experience with most of the following generic code, so
would appreciate a helping hand, whether that conceptually makes sense as
well as from the implementation point of view:
On 2023-07-25T23:52:06+0800, Chung-Lin Tang via Gcc-patches
wrote:
> On 2023/7/11 2:33 AM, Chung-Lin Tang via Gcc-patches wrote:
>> As we discussed earlier, the work for actually linking this to middle-end
>> points-to analysis is a somewhat non-trivial issue. This first patch allows
>> the language feature to be used in OpenACC directives first (with no effect
>> for now).
>> The middle-end changes are probably going to be a later patch.
>
> This second patch tries to link the readonly modifier to points-to analysis.
>
> There already exists SSA_NAME_POINTS_TO_READONLY_MEMORY and it's support in
> the
> alias oracle routines in tree-ssa-alias.cc, so basically what this patch does
> is
> try to make the variables holding the array section base pointers to have this
> flag set.
>
> There is an another OMP_CLAUSE_MAP_POINTS_TO_READONLY set by front-ends on the
> associated pointer clauses if OMP_CLAUSE_MAP_READONLY is set.
> Also a DECL_POINTS_TO_READONLY flag is set for VAR_DECLs when creating the tmp
> vars carrying these receiver references on the offloaded side. These
> eventually get translated to SSA_NAME_POINTS_TO_READONLY_MEMORY.
> This still doesn't always work as expected in terms of optimization:
> struct pointer fields and Fortran arrays (kind of like C structs) which have
> several accesses to create the pointer access on the receive/offloaded side,
> and SRA appears to not work on these sequences, so gets in the way of much
> redundancy elimination.
I understand correctly that this is left as future work? Please add the test
cases you have, XFAILed in some reasonable way.
> Currently have one testcase where we can demonstrate 'readonly' can avoid
> a clobber by function call.
:-)
> --- a/gcc/c/c-typeck.cc
> +++ b/gcc/c/c-typeck.cc
> @@ -14258,6 +14258,8 @@ handle_omp_array_sections (tree c, enum
> c_omp_region_type ort)
> OMP_CLAUSE_SET_MAP_KIND (c2, GOMP_MAP_ATTACH_DETACH);
>else
> OMP_CLAUSE_SET_MAP_KIND (c2, GOMP_MAP_FIRSTPRIVATE_POINTER);
> + if (OMP_CLAUSE_MAP_READONLY (c))
> + OMP_CLAUSE_MAP_POINTS_TO_READONLY (c2) = 1;
>OMP_CLAUSE_MAP_IMPLICIT (c2) = OMP_CLAUSE_MAP_IMPLICIT (c);
>if (OMP_CLAUSE_MAP_KIND (c2) != GOMP_MAP_FIRSTPRIVATE_POINTER
> && !c_mark_addressable (t))
> --- a/gcc/cp/semantics.cc
> +++ b/gcc/cp/semantics.cc
> @@ -5872,6 +5872,8 @@ handle_omp_array_sections (tree c, enum
> c_omp_region_type ort)
> }
> else
> OMP_CLAUSE_SET_MAP_KIND (c2, GOMP_MAP_FIRSTPRIVATE_POINTER);
> + if (OMP_CLAUSE_MAP_READONLY (c))
> + OMP_CLAUSE_MAP_POINTS_TO_READONLY (c2) = 1;
> OMP_CLAUSE_MAP_IMPLICIT (c2) = OMP_CLAUSE_MAP_IMPLICIT (c);
> if (OMP_CLAUSE_MAP_KIND (c2) != GOMP_MAP_FIRSTPRIVATE_POINTER
> && !cxx_mark_addressable (t))
> --- a/gcc/fortran/trans-openmp.cc
> +++ b/gcc/fortran/trans-openmp.cc
> @@ -2524,6 +2524,8 @@ gfc_trans_omp_array_s
