[Bug ada/68590] [6 Regression] FAIL: gnat.dg/loop_optimization19.adb scan-tree-dump-not optimized "Index_Check"

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68590

--- Comment #11 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Eric Botcazou from comment #10)
> > So the original compliant was that if we do that we pessimize code-gen
> > because we un-CSE a possibly large sub-expression.
> 
> Then maybe the "possibly large sub-expression" should be taken into account.
> We already try by means of skip_simple_arithmetic, but it's too simple.
> 
> > So you say that any tree-shared expr is only expanded once (as if it were 
> > wrapped in a save-expr)?
> 
> No, it will be reexpanded if it has no TREE_SIDE_EFFECTS.  What I'm saying
> is that you control it by means of TREE_SIDE_EFFECTS.
> 
> > Oh, so fold is performing CSE here?  Shouldn't _that_ folding not set
> > TREE_SIDE_EFFECTS on the SAVE_EXPR?  Instead of forcing it on every user...
> 
> It's not just fold, it's the way SAVE_EXPR works in GENERIC.  You need to be
> able to control when the variable is initialized at run time, otherwise it's
> a recipe for disaster.

Btw, removing TREE_SIDE_EFFECTS passes bootstrap for all languages but causes
(not double-checked whether they are really caused by the change but they
are new to my eyes):

                === acats tests ===
FAIL:   c64107a
FAIL:   cc3007b

                === g++ tests ===
FAIL: g++.dg/ext/vla9.C  -std=gnu++11 execution test
FAIL: g++.dg/ext/vla9.C  -std=gnu++14 execution test
FAIL: g++.dg/ext/vla9.C  -std=gnu++98 execution test
FAIL: g++.dg/pr58950.C  -std=gnu++11  (test for warnings, line 6)
FAIL: g++.dg/pr58950.C  -std=gnu++11 (test for excess errors)
FAIL: g++.dg/pr58950.C  -std=gnu++14  (test for warnings, line 6)
FAIL: g++.dg/pr58950.C  -std=gnu++14 (test for excess errors)
FAIL: g++.dg/pr58950.C  -std=gnu++98  (test for warnings, line 6)
FAIL: g++.dg/pr58950.C  -std=gnu++98 (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-3.c   -O0  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-3.c   -O1  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-3.c   -O2  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-3.c   -O2 -flto -fno-use-linker-plugin
-flto-partition=none  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-3.c   -O3 -g  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-3.c   -Os  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-4.c   -O0  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-4.c   -O1  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-4.c   -O2  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-4.c   -O2 -flto -fno-use-linker-plugin
-flto-partition=none  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-4.c   -O3 -g  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-4.c   -Os  (test for excess errors)

                === gcc tests ===
FAIL: gcc.dg/c99-bool-1.c execution test
FAIL: c-c++-common/gomp/atomic-12.c (test for excess errors)
FAIL: c-c++-common/gomp/atomic-13.c (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-3.c   -O0  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-3.c   -O1  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-3.c   -O2  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-3.c   -O2 -flto -fno-use-linker-plugin
-flto-partition=none  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-3.c   -O3 -g  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-3.c   -Os  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-4.c   -O0  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-4.c   -O1  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-4.c   -O2  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-4.c   -O2 -flto -fno-use-linker-plugin
-flto-partition=none  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-4.c   -O3 -g  (test for excess errors)
FAIL: c-c++-common/ubsan/save-expr-4.c   -Os  (test for excess errors)