This PR skips saving of any registers in main. Attribute OS_main can do this as well, however we can just drop any saves / restores in all optimized compilation -- not even the test suite needs these saves.
The feature can still be switched off by new -mno-OS_main Ok for trunk? gcc/ Don't save registers in main(). PR target/83737 * doc/invoke.texi (AVR Options) [-mOS_main]: Document it. * config/avr/avr.opt (-mOS_main): New target option. * config/avr/avr.c (avr_in_main_p): New static function. (avr_regs_to_save) [avr_in_main_p]: Return 0. (avr_prologue_setup_frame): Don't save any regs if avr_in_main_p. (avr_expand_epilogue): Same. * common/config/avr/avr-common.c (avr_option_optimization_table): Switch on -mOS_main for optimizing compilations.
Index: common/config/avr/avr-common.c =================================================================== --- common/config/avr/avr-common.c (revision 256338) +++ common/config/avr/avr-common.c (working copy) @@ -31,6 +31,7 @@ static const struct default_options avr_ // a frame without need when it tries to be smart around calls. { OPT_LEVELS_ALL, OPT_fcaller_saves, NULL, 0 }, { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_mgas_isr_prologues, NULL, 1 }, + { OPT_LEVELS_1_PLUS, OPT_mOS_main, NULL, 1 }, { OPT_LEVELS_NONE, 0, NULL, 0 } }; Index: config/avr/avr.c =================================================================== --- config/avr/avr.c (revision 256338) +++ config/avr/avr.c (working copy) @@ -1167,6 +1167,23 @@ avr_starting_frame_offset (void) } +/* Return true if we are supposed to be in main(). This is only used + to determine if the callee-saved registers don't need to be saved + because the caller of main (crt*.o) doesn't use any of them. */ + +static bool +avr_in_main_p () +{ + return (TARGET_OS_MAIN + && MAIN_NAME_P (DECL_NAME (current_function_decl)) + // FIXME: We'd like to also test `flag_hosted' which is only + // available in the C-ish fronts, so no such test for now. + // Instead, we test the return type of "main" which is not exactly + // the same but good enough. + && INTEGRAL_TYPE_P (TREE_TYPE (TREE_TYPE (current_function_decl)))); +} + + /* Return the number of hard registers to push/pop in the prologue/epilogue of the current function, and optionally store these registers in SET. */ @@ -1180,10 +1197,11 @@ avr_regs_to_save (HARD_REG_SET *set) CLEAR_HARD_REG_SET (*set); count = 0; - /* No need to save any registers if the function never returns or - has the "OS_task" or "OS_main" attribute. */ + /* No need to save any registers if the function never returns or has the + "OS_task" or "OS_main" attribute. Dito if we are in "main". */ if (TREE_THIS_VOLATILE (current_function_decl) + || avr_in_main_p() || cfun->machine->is_OS_task || cfun->machine->is_OS_main) return 0; @@ -1651,6 +1669,7 @@ avr_prologue_setup_frame (HOST_WIDE_INT && size < size_max && live_seq && !isr_p + && !avr_in_main_p() && !cfun->machine->is_OS_task && !cfun->machine->is_OS_main && !AVR_TINY); @@ -1713,7 +1732,9 @@ avr_prologue_setup_frame (HOST_WIDE_INT emit_push_byte (reg, true); if (frame_pointer_needed - && (!(cfun->machine->is_OS_task || cfun->machine->is_OS_main))) + && (!(avr_in_main_p() + || cfun->machine->is_OS_task + || cfun->machine->is_OS_main))) { /* Push frame pointer. Always be consistent about the ordering of pushes -- epilogue_restores expects the @@ -1834,6 +1855,9 @@ avr_prologue_setup_frame (HOST_WIDE_INT if (cfun->machine->is_interrupt) irq_state = 1; + /* IRQs might be on when entering "main", hence avr_in_main_p + is *not* included in the following test. */ + if (TARGET_NO_INTERRUPTS || cfun->machine->is_signal || cfun->machine->is_OS_main) @@ -2122,6 +2146,7 @@ avr_expand_epilogue (bool sibcall_p) && !isr_p && !cfun->machine->is_OS_task && !cfun->machine->is_OS_main + && !avr_in_main_p() && !AVR_TINY); if (minimize @@ -2227,7 +2252,9 @@ avr_expand_epilogue (bool sibcall_p) } /* size != 0 */ if (frame_pointer_needed - && !(cfun->machine->is_OS_task || cfun->machine->is_OS_main)) + && !(avr_in_main_p() + || cfun->machine->is_OS_task + || cfun->machine->is_OS_main)) { /* Restore previous frame_pointer. See avr_expand_prologue for rationale for not using pophi. */ Index: config/avr/avr.opt =================================================================== --- config/avr/avr.opt (revision 256338) +++ config/avr/avr.opt (working copy) @@ -70,6 +70,10 @@ Target Report Undocumented Mask(ORDER_1) morder2 Target Report Undocumented Mask(ORDER_2) +mOS_main +Target Report Mask(OS_MAIN) +Treat main as if it had attribute OS_main. + mtiny-stack Target Report Mask(TINY_STACK) Change only the low 8 bits of the stack pointer. Index: doc/invoke.texi =================================================================== --- doc/invoke.texi (revision 256338) +++ doc/invoke.texi (working copy) @@ -669,7 +669,8 @@ -imacros @var{file} -imultilib @var{dir -mbranch-cost=@var{cost} @gol -mcall-prologues -mgas-isr-prologues -mint8 @gol -mn_flash=@var{size} -mno-interrupts @gol --mrelax -mrmw -mstrict-X -mtiny-stack -mfract-convert-truncate @gol +-mOS_main -mrelax -mrmw -mstrict-X -mtiny-stack @gol +-mfract-convert-truncate @gol -mshort-calls -nodevicelib @gol -Waddr-space-convert -Wmisspelled-isr} @@ -16472,6 +16473,12 @@ Assume that the flash memory has a size Generated code is not compatible with hardware interrupts. Code size is smaller. +@item -mOS_main +@opindex mOS_main +Do not save registers in @code{main}. The effect is similar to +attaching attribute @ref{AVR Function Attributes,,@code{OS_main}} +to @code{main}. It is activated per default if optimization is on. + @item -mrelax @opindex mrelax Try to replace @code{CALL} resp.@: @code{JMP} instruction by the shorter