[PATCH v2] cselib: add function to check if SET is redundant [PR106187]
A SET operation that writes memory may have the same value as an earlier
store but if the alias sets of the new and earlier store do not conflict
then the set is not truly redundant. This can happen, for example, if
objects of different types share a stack slot.
To fix this we define a new function in cselib that first checks for
equality and if that is successful then finds the earlier store in the
value history and checks the alias sets.
The routine is used in two places elsewhere in the compiler. Firstly
in cfgcleanup and secondly in postreload.
gcc/ChangeLog:
* alias.h (mems_same_for_tbaa_p): Declare.
* alias.cc (mems_same_for_tbaa_p): New function.
* dse.cc (record_store): Use it instead of open-coding
alias check.
* cselib.h (cselib_redundant_set_p): Declare.
* cselib.cc: Include alias.h
(cselib_redundant_set_p): New function.
* cfgcleanup.cc: (mark_effect): Use cselib_redundant_set_p instead
of rtx_equal_for_cselib_p.
* postreload.c (reload_cse_simplify): Use cselib_redundant_set_p.
(reload_cse_noop_set_p): Delete.diff --git a/gcc/alias.cc b/gcc/alias.cc
index 8c08452e0ac..d54feb15268 100644
--- a/gcc/alias.cc
+++ b/gcc/alias.cc
@@ -389,6 +389,20 @@ refs_same_for_tbaa_p (tree earlier, tree later)
|| alias_set_subset_of (later_base_set, earlier_base_set));
}
+/* Similar to refs_same_for_tbaa_p() but for use on MEM rtxs. */
+bool
+mems_same_for_tbaa_p (rtx earlier, rtx later)
+{
+ gcc_assert (MEM_P (earlier));
+ gcc_assert (MEM_P (later));
+
+ return ((MEM_ALIAS_SET (earlier) == MEM_ALIAS_SET (later)
+ || alias_set_subset_of (MEM_ALIAS_SET (later),
+ MEM_ALIAS_SET (earlier)))
+ && (!MEM_EXPR (earlier)
+ || refs_same_for_tbaa_p (MEM_EXPR (earlier), MEM_EXPR (later;
+}
+
/* Returns a pointer to the alias set entry for ALIAS_SET, if there is
such an entry, or NULL otherwise. */
diff --git a/gcc/alias.h b/gcc/alias.h
index b2596518ac9..ee3db466763 100644
--- a/gcc/alias.h
+++ b/gcc/alias.h
@@ -40,6 +40,7 @@ tree reference_alias_ptr_type_1 (tree *);
bool alias_ptr_types_compatible_p (tree, tree);
int compare_base_decls (tree, tree);
bool refs_same_for_tbaa_p (tree, tree);
+bool mems_same_for_tbaa_p (rtx, rtx);
/* This alias set can be used to force a memory to conflict with all
other memories, creating a barrier across which no memory reference
diff --git a/gcc/cfgcleanup.cc b/gcc/cfgcleanup.cc
index 18047da7b24..a8b0139bb4d 100644
--- a/gcc/cfgcleanup.cc
+++ b/gcc/cfgcleanup.cc
@@ -208,7 +208,7 @@ mark_effect (rtx exp, regset nonequal)
return false;
case SET:
- if (rtx_equal_for_cselib_p (SET_DEST (exp), SET_SRC (exp)))
+ if (cselib_redundant_set_p (exp))
return false;
dest = SET_DEST (exp);
if (dest == pc_rtx)
diff --git a/gcc/cselib.cc b/gcc/cselib.cc
index 6769beeeaf8..6a5609786fa 100644
--- a/gcc/cselib.cc
+++ b/gcc/cselib.cc
@@ -32,6 +32,7 @@ along with GCC; see the file COPYING3. If not see
#include "dumpfile.h"
#include "cselib.h"
#include "function-abi.h"
+#include "alias.h"
/* A list of cselib_val structures. */
struct elt_list
@@ -1157,6 +1158,75 @@ rtx_equal_for_cselib_1 (rtx x, rtx y, machine_mode memmode, int depth)
return 1;
}
+/* Wrapper for rtx_equal_for_cselib_p to determine whether a SET is
+ truly redundant, taking into account aliasing information. */
+bool
+cselib_redundant_set_p (rtx set)
+{
+ gcc_assert (GET_CODE (set) == SET);
+ rtx dest = SET_DEST (set);
+ if (cselib_reg_set_mode (dest) != GET_MODE (dest))
+return false;
+
+ if (!rtx_equal_for_cselib_p (dest, SET_SRC (set)))
+return false;
+
+ while (GET_CODE (dest) == SUBREG
+ || GET_CODE (dest) == ZERO_EXTRACT
+ || GET_CODE (dest) == STRICT_LOW_PART)
+dest = XEXP (dest, 0);
+
+ if (!flag_strict_aliasing || !MEM_P (dest))
+return true;
+
+ /* For a store we need to check that suppressing it will not change
+ the effective alias set. */
+ rtx dest_addr = XEXP (dest, 0);
+
+ /* Lookup the equivalents to the original dest (rather than just the
+ MEM). */
+ cselib_val *src_val = cselib_lookup (SET_DEST (set),
+ GET_MODE (SET_DEST (set)),
+ 0, VOIDmode);
+
+ if (src_val)
+{
+ /* Walk the list of source equivalents to find the MEM accessing
+ the same location. */
+ for (elt_loc_list *l = src_val->locs; l; l = l->next)
+ {
+ rtx src_equiv = l->loc;
+ while (GET_CODE (src_equiv) == SUBREG
+ || GET_CODE (src_equiv) == ZERO_EXTRACT
+ || GET_CODE (src_equiv) == STRICT_LOW_PART)
+ src_equiv = XEXP (src_equiv, 0);
+
+ if (MEM_P (src_equiv))
+ {
+ /* Match the MEMs by comparing the addresses. We can
+ only remove the later store if the earlier aliases at
+ least all the accesses of the later one. */
+ if (rtx_equal_for_cselib_1 (dest_addr, XEXP (src_equiv, 0),
+ GET_MODE (dest), 0))
+ return mems_same_for_tbaa_p (src_equiv, d
Re: [PATCH v2] cselib: add function to check if SET is redundant [PR106187]
On 7/29/2022 7:52 AM, Richard Earnshaw via Gcc-patches wrote: A SET operation that writes memory may have the same value as an earlier store but if the alias sets of the new and earlier store do not conflict then the set is not truly redundant. This can happen, for example, if objects of different types share a stack slot. To fix this we define a new function in cselib that first checks for equality and if that is successful then finds the earlier store in the value history and checks the alias sets. The routine is used in two places elsewhere in the compiler. Firstly in cfgcleanup and secondly in postreload. gcc/ChangeLog: * alias.h (mems_same_for_tbaa_p): Declare. * alias.cc (mems_same_for_tbaa_p): New function. * dse.cc (record_store): Use it instead of open-coding alias check. * cselib.h (cselib_redundant_set_p): Declare. * cselib.cc: Include alias.h (cselib_redundant_set_p): New function. * cfgcleanup.cc: (mark_effect): Use cselib_redundant_set_p instead of rtx_equal_for_cselib_p. * postreload.c (reload_cse_simplify): Use cselib_redundant_set_p. (reload_cse_noop_set_p): Delete. Seems quite reasonable. The only question I would have would be whether or not you considered including the aliasing info into the hashing used by cselib. You'd probably still need the bulk of this patch as well since we could presumably still get a hash conflict with two stores of the same value to the same location, but with different alias sets (it's just much less likely), so perhaps it doesn't really buy us anything. Ideally this would include a testcase. You might be able to turn that non-executawble reduced case into something useful by scanning the post-reload dumps. Jeff
Re: [PATCH v2] cselib: add function to check if SET is redundant [PR106187]
On 30/07/2022 20:57, Jeff Law via Gcc-patches wrote: On 7/29/2022 7:52 AM, Richard Earnshaw via Gcc-patches wrote: A SET operation that writes memory may have the same value as an earlier store but if the alias sets of the new and earlier store do not conflict then the set is not truly redundant. This can happen, for example, if objects of different types share a stack slot. To fix this we define a new function in cselib that first checks for equality and if that is successful then finds the earlier store in the value history and checks the alias sets. The routine is used in two places elsewhere in the compiler. Firstly in cfgcleanup and secondly in postreload. gcc/ChangeLog: * alias.h (mems_same_for_tbaa_p): Declare. * alias.cc (mems_same_for_tbaa_p): New function. * dse.cc (record_store): Use it instead of open-coding alias check. * cselib.h (cselib_redundant_set_p): Declare. * cselib.cc: Include alias.h (cselib_redundant_set_p): New function. * cfgcleanup.cc: (mark_effect): Use cselib_redundant_set_p instead of rtx_equal_for_cselib_p. * postreload.c (reload_cse_simplify): Use cselib_redundant_set_p. (reload_cse_noop_set_p): Delete. Seems quite reasonable. The only question I would have would be whether or not you considered including the aliasing info into the hashing used by cselib. You'd probably still need the bulk of this patch as well since we could presumably still get a hash conflict with two stores of the same value to the same location, but with different alias sets (it's just much less likely), so perhaps it doesn't really buy us anything. I thought about this, but if the alias set were included in the hash, then surely you'd get every alias set in a different value. Then you'd miss the cases where the alias sets do conflict even though they are not the same. Anyway, the values /are/ the same so in some circumstances you might want to know that. Ideally this would include a testcase. You might be able to turn that non-executawble reduced case into something useful by scanning the post-reload dumps. I considered this as well, but the testcase I have is far too fragile, I think. The existing test only fails on Arm, only fails on 11.2 (not 11.3 or gcc-12 onwards), relies on two objects with the same value being in distinct alias sets but still assigned to the same stack slot and for some copy dance to end up trying to write back the original value to the same slot but with a non-conflicting set. And finally, the scheduler has to then try to move a load past the non-aliasing store. To get anywhere close to this I think we'd need something akin to the gimple reader but for RTL so that we could set up all the conditions for the failure without the risk of an earlier transform blowing the test away. I even considered whether we could start with a gimple dump and bypassing all the tree/gimple transformations, but even that would be still at the mercy of the stack-slot allocation algorithm. Jeff R.
Re: [PATCH v2] cselib: add function to check if SET is redundant [PR106187]
On 01/08/2022 11:38, Richard Earnshaw via Gcc-patches wrote: On 30/07/2022 20:57, Jeff Law via Gcc-patches wrote: On 7/29/2022 7:52 AM, Richard Earnshaw via Gcc-patches wrote: A SET operation that writes memory may have the same value as an earlier store but if the alias sets of the new and earlier store do not conflict then the set is not truly redundant. This can happen, for example, if objects of different types share a stack slot. To fix this we define a new function in cselib that first checks for equality and if that is successful then finds the earlier store in the value history and checks the alias sets. The routine is used in two places elsewhere in the compiler. Firstly in cfgcleanup and secondly in postreload. gcc/ChangeLog: * alias.h (mems_same_for_tbaa_p): Declare. * alias.cc (mems_same_for_tbaa_p): New function. * dse.cc (record_store): Use it instead of open-coding alias check. * cselib.h (cselib_redundant_set_p): Declare. * cselib.cc: Include alias.h (cselib_redundant_set_p): New function. * cfgcleanup.cc: (mark_effect): Use cselib_redundant_set_p instead of rtx_equal_for_cselib_p. * postreload.c (reload_cse_simplify): Use cselib_redundant_set_p. (reload_cse_noop_set_p): Delete. Seems quite reasonable. The only question I would have would be whether or not you considered including the aliasing info into the hashing used by cselib. You'd probably still need the bulk of this patch as well since we could presumably still get a hash conflict with two stores of the same value to the same location, but with different alias sets (it's just much less likely), so perhaps it doesn't really buy us anything. I thought about this, but if the alias set were included in the hash, then surely you'd get every alias set in a different value. Then you'd miss the cases where the alias sets do conflict even though they are not the same. Anyway, the values /are/ the same so in some circumstances you might want to know that. Ideally this would include a testcase. You might be able to turn that non-executawble reduced case into something useful by scanning the post-reload dumps. I considered this as well, but the testcase I have is far too fragile, I think. The existing test only fails on Arm, only fails on 11.2 (not 11.3 or gcc-12 onwards), relies on two objects with the same value being in distinct alias sets but still assigned to the same stack slot and for some copy dance to end up trying to write back the original value to the same slot but with a non-conflicting set. And finally, the scheduler has to then try to move a load past the non-aliasing store. To get anywhere close to this I think we'd need something akin to the gimple reader but for RTL so that we could set up all the conditions for the failure without the risk of an earlier transform blowing the test away. I wasn't aware of the rtl reader already in the compiler. But it doesn't really get me any closer as it is lacking in so many regards: - It can't handle (const_double:SF ...) - it tries to handle the argument as an int. This is a consequence, I think, of the reader being based on that for reading machine descriptions where FP const_double is simply never encountered. - It doesn't seem to handle anything much more than very basic types, and in particular appears to have no way of ensuring that alias sets match up with the type system. I even considered whether we could start with a gimple dump and bypassing all the tree/gimple transformations, but even that would be still at the mercy of the stack-slot allocation algorithm. I spent a while trying to get some gimple out of the dumpers in a form that was usable, but that's pretty much a non-starter. To make it work we'd need to add support for gimple clobbers on objects - without that there's no way to get the stack-slot sharing code to work. Furthermore, even feeding fully-optimized gimple directly into expand is such a long way from the postreload pass, that I can't believe the testcase would remain stable for long. And the other major issue is that the original testcase is heavily templated C++ and neither of the parsers gimple or rtl is supported in cc1plus: converting the boilerplate to be C-friendly is probably going to be hard. I can't afford to spend much more time on this, especially given the low-quality test we're going to get out of the end of the process. Jeff R. R.
Re: [PATCH v2] cselib: add function to check if SET is redundant [PR106187]
On 8/2/2022 10:06 AM, Richard Earnshaw wrote: On 01/08/2022 11:38, Richard Earnshaw via Gcc-patches wrote: On 30/07/2022 20:57, Jeff Law via Gcc-patches wrote: On 7/29/2022 7:52 AM, Richard Earnshaw via Gcc-patches wrote: A SET operation that writes memory may have the same value as an earlier store but if the alias sets of the new and earlier store do not conflict then the set is not truly redundant. This can happen, for example, if objects of different types share a stack slot. To fix this we define a new function in cselib that first checks for equality and if that is successful then finds the earlier store in the value history and checks the alias sets. The routine is used in two places elsewhere in the compiler. Firstly in cfgcleanup and secondly in postreload. gcc/ChangeLog: * alias.h (mems_same_for_tbaa_p): Declare. * alias.cc (mems_same_for_tbaa_p): New function. * dse.cc (record_store): Use it instead of open-coding alias check. * cselib.h (cselib_redundant_set_p): Declare. * cselib.cc: Include alias.h (cselib_redundant_set_p): New function. * cfgcleanup.cc: (mark_effect): Use cselib_redundant_set_p instead of rtx_equal_for_cselib_p. * postreload.c (reload_cse_simplify): Use cselib_redundant_set_p. (reload_cse_noop_set_p): Delete. Seems quite reasonable. The only question I would have would be whether or not you considered including the aliasing info into the hashing used by cselib. You'd probably still need the bulk of this patch as well since we could presumably still get a hash conflict with two stores of the same value to the same location, but with different alias sets (it's just much less likely), so perhaps it doesn't really buy us anything. I thought about this, but if the alias set were included in the hash, then surely you'd get every alias set in a different value. Then you'd miss the cases where the alias sets do conflict even though they are not the same. Anyway, the values /are/ the same so in some circumstances you might want to know that. Ideally this would include a testcase. You might be able to turn that non-executawble reduced case into something useful by scanning the post-reload dumps. I considered this as well, but the testcase I have is far too fragile, I think. The existing test only fails on Arm, only fails on 11.2 (not 11.3 or gcc-12 onwards), relies on two objects with the same value being in distinct alias sets but still assigned to the same stack slot and for some copy dance to end up trying to write back the original value to the same slot but with a non-conflicting set. And finally, the scheduler has to then try to move a load past the non-aliasing store. To get anywhere close to this I think we'd need something akin to the gimple reader but for RTL so that we could set up all the conditions for the failure without the risk of an earlier transform blowing the test away. I wasn't aware of the rtl reader already in the compiler. But it doesn't really get me any closer as it is lacking in so many regards: - It can't handle (const_double:SF ...) - it tries to handle the argument as an int. This is a consequence, I think, of the reader being based on that for reading machine descriptions where FP const_double is simply never encountered. - It doesn't seem to handle anything much more than very basic types, and in particular appears to have no way of ensuring that alias sets match up with the type system. I even considered whether we could start with a gimple dump and bypassing all the tree/gimple transformations, but even that would be still at the mercy of the stack-slot allocation algorithm. I spent a while trying to get some gimple out of the dumpers in a form that was usable, but that's pretty much a non-starter. To make it work we'd need to add support for gimple clobbers on objects - without that there's no way to get the stack-slot sharing code to work. Furthermore, even feeding fully-optimized gimple directly into expand is such a long way from the postreload pass, that I can't believe the testcase would remain stable for long. And the other major issue is that the original testcase is heavily templated C++ and neither of the parsers gimple or rtl is supported in cc1plus: converting the boilerplate to be C-friendly is probably going to be hard. I can't afford to spend much more time on this, especially given the low-quality test we're going to get out of the end of the process. Understood. Let's just go with the patch as-is. That's normal for cases where we can't produce a reasonable test. jeff
Re: [PATCH v2] cselib: add function to check if SET is redundant [PR106187]
On 03/08/2022 00:36, Jeff Law wrote: On 8/2/2022 10:06 AM, Richard Earnshaw wrote: On 01/08/2022 11:38, Richard Earnshaw via Gcc-patches wrote: On 30/07/2022 20:57, Jeff Law via Gcc-patches wrote: On 7/29/2022 7:52 AM, Richard Earnshaw via Gcc-patches wrote: A SET operation that writes memory may have the same value as an earlier store but if the alias sets of the new and earlier store do not conflict then the set is not truly redundant. This can happen, for example, if objects of different types share a stack slot. To fix this we define a new function in cselib that first checks for equality and if that is successful then finds the earlier store in the value history and checks the alias sets. The routine is used in two places elsewhere in the compiler. Firstly in cfgcleanup and secondly in postreload. gcc/ChangeLog: * alias.h (mems_same_for_tbaa_p): Declare. * alias.cc (mems_same_for_tbaa_p): New function. * dse.cc (record_store): Use it instead of open-coding alias check. * cselib.h (cselib_redundant_set_p): Declare. * cselib.cc: Include alias.h (cselib_redundant_set_p): New function. * cfgcleanup.cc: (mark_effect): Use cselib_redundant_set_p instead of rtx_equal_for_cselib_p. * postreload.c (reload_cse_simplify): Use cselib_redundant_set_p. (reload_cse_noop_set_p): Delete. Seems quite reasonable. The only question I would have would be whether or not you considered including the aliasing info into the hashing used by cselib. You'd probably still need the bulk of this patch as well since we could presumably still get a hash conflict with two stores of the same value to the same location, but with different alias sets (it's just much less likely), so perhaps it doesn't really buy us anything. I thought about this, but if the alias set were included in the hash, then surely you'd get every alias set in a different value. Then you'd miss the cases where the alias sets do conflict even though they are not the same. Anyway, the values /are/ the same so in some circumstances you might want to know that. Ideally this would include a testcase. You might be able to turn that non-executawble reduced case into something useful by scanning the post-reload dumps. I considered this as well, but the testcase I have is far too fragile, I think. The existing test only fails on Arm, only fails on 11.2 (not 11.3 or gcc-12 onwards), relies on two objects with the same value being in distinct alias sets but still assigned to the same stack slot and for some copy dance to end up trying to write back the original value to the same slot but with a non-conflicting set. And finally, the scheduler has to then try to move a load past the non-aliasing store. To get anywhere close to this I think we'd need something akin to the gimple reader but for RTL so that we could set up all the conditions for the failure without the risk of an earlier transform blowing the test away. I wasn't aware of the rtl reader already in the compiler. But it doesn't really get me any closer as it is lacking in so many regards: - It can't handle (const_double:SF ...) - it tries to handle the argument as an int. This is a consequence, I think, of the reader being based on that for reading machine descriptions where FP const_double is simply never encountered. - It doesn't seem to handle anything much more than very basic types, and in particular appears to have no way of ensuring that alias sets match up with the type system. I even considered whether we could start with a gimple dump and bypassing all the tree/gimple transformations, but even that would be still at the mercy of the stack-slot allocation algorithm. I spent a while trying to get some gimple out of the dumpers in a form that was usable, but that's pretty much a non-starter. To make it work we'd need to add support for gimple clobbers on objects - without that there's no way to get the stack-slot sharing code to work. Furthermore, even feeding fully-optimized gimple directly into expand is such a long way from the postreload pass, that I can't believe the testcase would remain stable for long. And the other major issue is that the original testcase is heavily templated C++ and neither of the parsers gimple or rtl is supported in cc1plus: converting the boilerplate to be C-friendly is probably going to be hard. I can't afford to spend much more time on this, especially given the low-quality test we're going to get out of the end of the process. Understood. Let's just go with the patch as-is. That's normal for cases where we can't produce a reasonable test. Thanks, committed to trunk. Will work on backports if it doesn't throw up any issues in the next few days. R. jeff
