On Wed, Sep 30, 2026 at 12:57 AM Richard Biener <[email protected]> wrote:
>
> The following adds gimple_could_raise_ieee_exception_p and honors it
> in DCE and DSE.
>
> This shows, for example in gcc.dg/tree-ssa/fp-trapping-cmp-4.c,
> that some passes fail to clean up after themselves when they
> no longer can rely on DCE.
Yes it looks like phiopt does not cleanup after itself
(r17-2709-gde8be0eb05d951 added the code).
I can try to fix that if still needed. It looks like depending on
which bb was removed; phiopt didn't need to cleanup after itself which
is why I didn't notice that until now.
>
> Bootstrapped and tested on x86_64-unknown-linux-gnu. I think we
> shouldn't do something like this without also swapping the
> default of -ftrapping-math. Full testsuite fallout is
>
> +FAIL: g++.dg/opt/pr94589-5.C -std=gnu++20 scan-tree-dump-times
> optimized "[ij
> ]_[0-9]+\\\\(D\\\\) (?:<|<=|>|>=) [ij]_[0-9]+\\\\(D\\\\)" 8
> +FAIL: gcc.dg/pr94589-5.c scan-tree-dump-times optimized
> "[ij]_[0-9]+\\\\(D\\\\) (?:<|<=|>|>=) [ij]_[0-9]+\\\\(D\\\\)" 24
> +FAIL: gcc.dg/pr94589-5.c scan-tree-dump-times optimized
> "i_[0-9]+\\\\(D\\\\) (?:<|<=|>|>=) 5\\\\.0" 24
> +FAIL: gcc.dg/tree-ssa/fp-trapping-cmp-4.c scan-tree-dump-times optimized
> " < " 1
> +FAIL: gcc.dg/tree-ssa/fp-trapping-cmp-5.c scan-tree-dump-times optimized
> " < " 1
> +FAIL: gcc.dg/tree-ssa/fp-trapping-cmp-8.c scan-tree-dump dce3
> "a_[0-9]+\\\\(D\\\\) > b_[0-9]+\\\\(D\\\\)"
> +FAIL: gcc.dg/tree-ssa/pr125385.c scan-tree-dump-not optimized
> "\\\\(float\\\\)"
gcc.dg/tree-ssa/pr125385.c is a missed optimization though as we know
the comparisons there can't trap. Due to the ranges not including
NaNs.
> +FAIL: gcc.dg/tree-ssa/sincos-ice-on-ifn_sin-call.c (internal compiler
> error: in expand_fn_using_insn, at internal-fn.cc:270)
> +FAIL: gcc.dg/tree-ssa/sincos-ice-on-ifn_sin-call.c (test for excess
> errors)
> +FAIL: gcc.dg/tree-ssa/tailcall-9.c scan-tree-dump tailc "Found tail call"
> +FAIL: gcc.dg/vect/bb-slp-48.c -flto -ffat-lto-objects
> scan-tree-dump-times optimized " \\\\+ " 4
> +FAIL: gcc.dg/vect/bb-slp-48.c scan-tree-dump-times optimized " \\\\+ " 4
> +FAIL: gcc.target/i386/opt-1.c scan-assembler prefetcht0
> +FAIL: gcc.target/i386/opt-2.c scan-assembler prefetcht0
> +FAIL: gcc.target/i386/pr92645-3.c scan-tree-dump-times cddce1
> "BIT_FIELD_REF" 4
>
> I'll note that, of course, RTL DCE will still elide the instructions
> (so does simple-DCE on GIMPLE). Preserving the stmts won't help
> with reliable access through the FP environment (one more option for
> a -fno-trapping-math default). Maybe we can really go forward with
> -fno-trapping-math by default for GCC 17?
>
> Richard.
>
> PR tree-optimization/53805
> * gimple.h (gimple_could_raise_ieee_exception_p): Declare.
> * gimple.cc (gimple_could_raise_ieee_exception_p): New function.
> * tree-ssa-dce.cc (mark_stmt_if_obviously_necessary):
> Statements that might raise IEEE exceptions are necessary.
> * tree-ssa-dse.cc (pass_dse::execute): Likewise.
>
> * gcc.dg/tree-ssa/fp-trapping-cmp-9.c: New testcase.
> ---
> gcc/gimple.cc | 32 +++++++++++++++++++
> gcc/gimple.h | 1 +
> .../gcc.dg/tree-ssa/fp-trapping-cmp-9.c | 9 ++++++
> gcc/tree-ssa-dce.cc | 7 ++++
> gcc/tree-ssa-dse.cc | 3 +-
> 5 files changed, 51 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/fp-trapping-cmp-9.c
>
> diff --git a/gcc/gimple.cc b/gcc/gimple.cc
> index 04ad17f5955..72a7e3f9603 100644
> --- a/gcc/gimple.cc
> +++ b/gcc/gimple.cc
> @@ -2493,6 +2493,38 @@ gimple_assign_rhs_could_trap_p (gimple *s)
> return gimple_could_trap_p_1 (s, true, false);
> }
>
> +/* Return true if STMT could raise a IEEE exception. */
> +
> +bool
> +gimple_could_raise_ieee_exception_p (const gimple *stmt)
> +{
> + if (!flag_trapping_math)
> + return false;
> +
> + switch (gimple_code (stmt))
> + {
> + case GIMPLE_ASSIGN:
> + {
> + enum tree_code code = gimple_assign_rhs_code (stmt);
> + return (!gimple_assign_single_p (stmt)
> + && code != COND_EXPR
> + && FLOAT_TYPE_P
> + (TREE_TYPE (TREE_CODE_CLASS (code) == tcc_comparison
> + ? gimple_assign_rhs1 (stmt)
> + : gimple_assign_lhs (stmt)))
> + && operation_could_trap_p (code, true, false, NULL_TREE));
> + }
> + case GIMPLE_COND:
> + return (FLOAT_TYPE_P (TREE_TYPE (gimple_cond_lhs (stmt)))
> + && operation_could_trap_p (gimple_cond_code (stmt), true, false,
> + NULL_TREE));
> + default:
> + /* ??? This is incomplete, gcall and gasm can as well. */
> + ;
> + }
> +
> + return false;
> +}
>
> /* Print debugging information for gimple stmts generated. */
>
> diff --git a/gcc/gimple.h b/gcc/gimple.h
> index 82045c44bbe..37a86061c48 100644
> --- a/gcc/gimple.h
> +++ b/gcc/gimple.h
> @@ -1652,6 +1652,7 @@ void gimple_move_vops (gimple *, gimple *);
> bool gimple_has_side_effects (const gimple *);
> bool gimple_could_trap_p_1 (const gimple *, bool, bool);
> bool gimple_could_trap_p (const gimple *);
> +bool gimple_could_raise_ieee_exception_p (const gimple *);
> bool gimple_assign_rhs_could_trap_p (gimple *);
> extern void dump_gimple_statistics (void);
> unsigned get_gimple_rhs_num_ops (enum tree_code);
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/fp-trapping-cmp-9.c
> b/gcc/testsuite/gcc.dg/tree-ssa/fp-trapping-cmp-9.c
> new file mode 100644
> index 00000000000..b27386408ca
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/fp-trapping-cmp-9.c
> @@ -0,0 +1,9 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O3 -fdump-tree-optimized" } */
> +
> +void f(double a,double b)
> +{
> + _Bool c = a>b;
> +}
> +
> +/* { dg-final { scan-tree-dump "a_\[0-9\]\+\\\(D\\\) > b_\[0-9\]\+\\\(D\\\)"
> "optimized" } } */
> diff --git a/gcc/tree-ssa-dce.cc b/gcc/tree-ssa-dce.cc
> index a5391290c4d..c511177c16d 100644
> --- a/gcc/tree-ssa-dce.cc
> +++ b/gcc/tree-ssa-dce.cc
> @@ -503,6 +503,13 @@ mark_stmt_if_obviously_necessary (gimple *stmt, bool
> aggressive)
> return;
> }
>
> + /* Preserve statements possibly setting IEEE exceptions. */
> + if (gimple_could_raise_ieee_exception_p (stmt))
> + {
> + mark_stmt_necessary (stmt, true);
> + return;
> + }
> +
> if ((gimple_vdef (stmt) && keep_all_vdefs_p ())
> || stmt_may_clobber_global_p (stmt, false))
> {
> diff --git a/gcc/tree-ssa-dse.cc b/gcc/tree-ssa-dse.cc
> index 0623f2c9090..62ccd87c036 100644
> --- a/gcc/tree-ssa-dse.cc
> +++ b/gcc/tree-ssa-dse.cc
> @@ -1732,7 +1732,8 @@ pass_dse::execute (function *fun)
> && !gimple_has_side_effects (stmt)
> && !is_ctrl_altering_stmt (stmt)
> && (!stmt_could_throw_p (fun, stmt)
> - || fun->can_delete_dead_exceptions))
> + || fun->can_delete_dead_exceptions)
> + && !gimple_could_raise_ieee_exception_p (stmt))
> {
> if (dump_file && (dump_flags & TDF_DETAILS))
> {
> --
> 2.51.0
