This patch would like to fix the wroing code generation for the scalar signed SAT_SUB. The input can be QI/HI/SI/DI while the alu like sub can only work on Xmode. Unfortunately we don't have sub/add for non-Xmode like QImode in scalar, thus we need to sign extend to Xmode to ensure we have the correct value before ALU like sub. The gen_lowpart will generate something like lbu which has all zero for highest bits. For example, when 0xff(-1 for QImode) sub 0x1(1 for QImode), we actually want to -1 - 1 = -2, but if there is no sign extend like lbu, we will get 0xff - 1 = 0xfe which is incorrect. Thus, we have to sign extend 0xff(Qmode) to 0xffffffffffffffff(assume XImode is DImode) before sub in Xmode. The below test suites are passed for this patch. * The rv64gcv fully regression test. PR target/117688 gcc/ChangeLog: * config/riscv/riscv.cc (riscv_expand_sssub): Leverage the helper riscv_extend_to_xmode_reg with SIGN_EXTEND. gcc/testsuite/ChangeLog: * gcc.target/riscv/pr117688.h: Add test helper macro. * gcc.target/riscv/pr117688-sub-run-1-s16.c: New test. * gcc.target/riscv/pr117688-sub-run-1-s32.c: New test. * gcc.target/riscv/pr117688-sub-run-1-s64.c: New test. * gcc.target/riscv/pr117688-sub-run-1-s8.c: New test. Signed-off-by: Pan Li <pan2.li@intel.com>