On 6/4/25 9:40 AM, Xi Ruoyao wrote:
On Wed, 2025-05-28 at 18:17 +0100, Richard Sandiford wrote:
Sorry for the slow reply, had a few days off.
Xi Ruoyao <xry...@xry111.site> writes:
If we see a promoted subreg and TRULY_NOOP_TRUNCATION says the
truncation is not a noop, then all bits of the inner reg are live. We
cannot reduce the live mask to that of the mode of the subreg.
gcc/ChangeLog:
PR rtl-optimization/120050
* ext-dce.cc (ext_dce_process_uses): Break early if a SUBREG in
rhs is promoted and the truncation from the inner mode to the
outer mode is not a noop when handling SETs.
---
Bootstrapped on mips64el-linux-gnuabi64. Ok for trunk?
gcc/ext-dce.cc | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/gcc/ext-dce.cc b/gcc/ext-dce.cc
index a0343950141..3b21e68b90c 100644
--- a/gcc/ext-dce.cc
+++ b/gcc/ext-dce.cc
@@ -35,6 +35,7 @@ along with GCC; see the file COPYING3. If not see
#include "print-rtl.h"
#include "dbgcnt.h"
#include "diagnostic-core.h"
+#include "target.h"
/* These should probably move into a C++ class. */
static vec<bitmap_head> livein;
@@ -764,13 +765,20 @@ ext_dce_process_uses (rtx_insn *insn, rtx obj,
We don't want to mark those bits live unnecessarily
as that inhibits extension elimination in important
cases such as those in Coremark. So we need that
- outer code. */
+ outer code.
+
+ But if !TRULY_NOOP_TRUNCATION_MODES_P, those bits
+ may be actually alive with any promoted subreg
+ regardless of the outer code. See PR 120050. */
How about expanding on this a bit:
If !TRULY_NOOP_TRUNCATION_MODES_P holds true for
the subreg, then the mode change performed by Y
would normally need to be a TRUNCATE rather than
a SUBREG. It is probably the guarantee provided
by SUBREG_PROMOTED_VAR_P that allows the SUBREG
in Y as an exception. We must therefore preserve
that guarantee and treat the upper bits of the
inner register as live regardless of the outer code.
See PR 120050. */
OK with that change, thanks.
Pushed r16-1110 with the comment updated.
Will wait for a few days before backporting to 15.
Thanks for taking care of this. I've been slammed with some personal
issues recently.
jeff
