[gcc r15-10304] AVR: Disable tree-switch-conversion per default.

[Georg-Johann Lay]

https://gcc.gnu.org/g:29ef01494e39110344d36e5df0918761a6a8c92e

commit r15-10304-g29ef01494e39110344d36e5df0918761a6a8c92e
Author: Georg-Johann Lay <avr@gjlay.de>
Date:   Sat Sep 6 13:38:48 2025 +0200

    AVR: Disable tree-switch-conversion per default.
    
    There are at least two cases where tree-switch-conversion leads
    to unpleasant resource allocation:
    
    PR49857
        The lookup table lives in RAM.  This is the case for all
        devices that locate .rodata in RAM, which is for almost
        all AVR devices.
    
    PR81540
        Code is bloated for 64-bit inputs.
    
    As far as PR49857 is concerned, a target hook that may add an
    address-space qualifier to the lookup table is the obvious
    solution, though a respective patch has always been rejected by
    global maintainers for non-technical reasons.
    
    gcc/
            PR target/81540
            PR target/49857
            * common/config/avr/avr-common.cc: Disable -ftree-switch-conversion.
    
    (cherry picked from commit 912159d2b5429c3126756b56723dd4f32dd56bdd)

Diff:
---
 gcc/common/config/avr/avr-common.cc | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/gcc/common/config/avr/avr-common.cc b/gcc/common/config/avr/avr-common.cc
index d8b982c4fa6f..914b7417fdad 100644
--- a/gcc/common/config/avr/avr-common.cc
+++ b/gcc/common/config/avr/avr-common.cc
@@ -29,6 +29,13 @@
 /* Implement TARGET_OPTION_OPTIMIZATION_TABLE.  */
 static const struct default_options avr_option_optimization_table[] =
   {
+    // The lookup table from tree-swicth-conversion.cc lives in .rodata
+    // and hence consumes RAM on almost all devices.  As PR49857 has
+    // been rejected several times for non-technical reasons, just
+    // disable -ftree-switch-conversion by default.  But even with PR49857
+    // in place there remains PR81540, which cannot be filtered out since
+    // the pass has no way to hook in.
+    { OPT_LEVELS_ALL, OPT_ftree_switch_conversion, NULL, 0 },
     // The only effect of -fcaller-saves might be that it triggers
     // a frame without need when it tries to be smart around calls.
     { OPT_LEVELS_ALL, OPT_fcaller_saves, NULL, 0 },