On Mon, Oct 13, 2025 at 11:28 PM Richard Biener <[email protected]> wrote: > > On Tue, Oct 14, 2025 at 5:35 AM Andrew Pinski > <[email protected]> wrote: > > > > When DCE removes the lhs of __builtin_stack_save, we can also > > remove the function itself. This adds that to gimple fold. > > > > Bootstrapped and tested on x86_64-linux-gnu. > > Huh, this makes me wonder why it's so difficult to remove the whole stmt in > DCE > rather than just the LHS? DCE-by-replacing-with-nop is a bit ugly.
The problem is in fold_stmt, you can't remove the current statement because all callers think fold_stmt will stay the current statement. Changing that is much harder. This pattern of replacing with gimple_build_nop is used all over gimple-fold.cc already. See gimple_fold_builtin_memory_op, replace_call_with_value and gimplify_and_update_call_from_tree for examples of where this has been there before. Thanks, Andrew Pinski > > > PR tree-optimization/122037 > > > > gcc/ChangeLog: > > > > * gimple-fold.cc (gimple_fold_builtin_stack_save): New function. > > (gimple_fold_builtin): Call gimple_fold_builtin_stack_save > > for __builtin_stack_save. > > > > gcc/testsuite/ChangeLog: > > > > * gcc.dg/tree-ssa/vla-1.c: New test. > > > > Signed-off-by: Andrew Pinski <[email protected]> > > --- > > gcc/gimple-fold.cc | 21 +++++++++++++++++++++ > > gcc/testsuite/gcc.dg/tree-ssa/vla-1.c | 14 ++++++++++++++ > > 2 files changed, 35 insertions(+) > > create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/vla-1.c > > > > diff --git a/gcc/gimple-fold.cc b/gcc/gimple-fold.cc > > index edcc04adc08..5f88cce7760 100644 > > --- a/gcc/gimple-fold.cc > > +++ b/gcc/gimple-fold.cc > > @@ -5372,6 +5372,25 @@ gimple_fold_builtin_stdarg (gimple_stmt_iterator > > *gsi, gcall *call) > > } > > } > > > > +/* Fold __builtin_stack_save if possible. Returns true if it was folded. > > + Currently this removes the call if the lhs is unused. */ > > +static bool > > +gimple_fold_builtin_stack_save (gimple_stmt_iterator *gsi, gcall *call) > > +{ > > + if (gimple_call_num_args (call) != 0) > > + return false; > > + > > + if (gimple_call_lhs (call)) > > + return false; > > + > > + // Remove the call since the result is unused. > > + unlink_stmt_vdef (call); > > + release_defs (call); > > + gsi_replace (gsi, gimple_build_nop (), true); > > + > > + return true; > > +} > > + > > /* Fold the non-target builtin at *GSI and return whether any > > simplification > > was made. */ > > > > @@ -5390,6 +5409,8 @@ gimple_fold_builtin (gimple_stmt_iterator *gsi) > > enum built_in_function fcode = DECL_FUNCTION_CODE (callee); > > switch (fcode) > > { > > + case BUILT_IN_STACK_SAVE: > > + return gimple_fold_builtin_stack_save (gsi, stmt); > > case BUILT_IN_VA_START: > > case BUILT_IN_VA_END: > > case BUILT_IN_VA_COPY: > > diff --git a/gcc/testsuite/gcc.dg/tree-ssa/vla-1.c > > b/gcc/testsuite/gcc.dg/tree-ssa/vla-1.c > > new file mode 100644 > > index 00000000000..ec19babebc4 > > --- /dev/null > > +++ b/gcc/testsuite/gcc.dg/tree-ssa/vla-1.c > > @@ -0,0 +1,14 @@ > > +/* { dg-do compile } */ > > +/* { dg-options "-O2 -fdump-tree-optimized" } */ > > +/* PR tree-optimization/122037 */ > > + > > +void bar1 (char *, int) __attribute__((noreturn)); > > +void foo1 (int size) > > +{ > > + char temp[size]; > > + temp[size-1] = '\0'; > > + bar1 (temp, size); > > +} > > + > > +/* The call to __builtin_stack_save should have been removed. */ > > +/* { dg-final { scan-tree-dump-not "__builtin_stack_save " "optimized" } } > > */ > > -- > > 2.43.0 > >
