https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127306
Bug ID: 127306
Summary: RISC-V bclr_lowest_set_bit pass produces wrong code
Product: gcc
Version: 16.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: uwu at icenowy dot me
Target Milestone: ---
Created attachment 65547
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=65547&action=edit
A minimal reproducer
With Zbb/Zbs extensions enabled and an optimization level higher than
(including) O1, GCC 16 produces wrong code for the program attached. (0xffff
should be the expected result)
```
icenowy@edelgard [ tmp ] $ /opt/or-cross/a23/bin/riscv64-openruyi-linux-gcc
gcc-riscv-bclr-miscompile.c -O2 -march=rv64gc
icenowy@edelgard [ tmp ] $ qemu-riscv64-static -L /opt/or-cross/a23/sysroot
./a.out
0xffff
icenowy@edelgard [ tmp ] $ /opt/or-cross/a23/bin/riscv64-openruyi-linux-gcc
gcc-riscv-bclr-miscompile.c -O2 -march=rv64gc_zbb_zbs
icenowy@edelgard [ tmp ] $ qemu-riscv64-static -L /opt/or-cross/a23/sysroot
./a.out
0x2aab
icenowy@edelgard [ tmp ] ! /opt/or-cross/a23/bin/riscv64-openruyi-linux-gcc
gcc-riscv-bclr-miscompile.c -O0 -march=rv64gc_zbb_zbs
icenowy@edelgard [ tmp ] $ qemu-riscv64-static -L /opt/or-cross/a23/sysroot
./a.out
0xffff
icenowy@edelgard [ tmp ] $ gcc gcc-riscv-bclr-miscompile.c
icenowy@edelgard [ tmp ] $ ./a.out
0xffff
```
Investigation shows that disabling the `bclr_lowest_set_bit` pass will fix this
issue, which indicates that this pass is faulty.
