https://gcc.gnu.org/g:5163cf2ae14c5e7ec730ad72680564001d0d0441
commit r15-7373-g5163cf2ae14c5e7ec730ad72680564001d0d0441
Author: Richard Earnshaw <rearn...@arm.com>
Date: Thu Dec 19 16:00:48 2024 +0000
arm: Use POP {pc} to return when returning [PR118089]
When generating thumb2 code,
LDM SP!, {PC}
is a two-byte instruction, whereas
LDR PC, [SP], #4
is needs 4 bytes. When optimizing for size, or when there's no obvious
performance benefit prefer the former.
gcc/ChangeLog:
PR target/118089
* config/arm/arm.cc (thumb2_expand_return): Use LDM SP!, {PC}
when optimizing for size, or when there's no performance benefit
over
LDR PC, [SP], #4.
(arm_expand_epilogue): Likewise.
Diff:
---
gcc/config/arm/arm.cc | 62 +++++++++++++++++++++++++++++----------------------
1 file changed, 35 insertions(+), 27 deletions(-)
diff --git a/gcc/config/arm/arm.cc b/gcc/config/arm/arm.cc
index 4ee84361dc6e..7e2082101d83 100644
--- a/gcc/config/arm/arm.cc
+++ b/gcc/config/arm/arm.cc
@@ -27762,35 +27762,40 @@ thumb2_expand_return (bool simple_return)
/* TODO: Verify that this path is never taken for cmse_nonsecure_entry
functions or adapt code to handle according to ACLE. This path should
not be reachable for cmse_nonsecure_entry functions though we prefer
- to assert it for now to ensure that future code changes do not silently
- change this behavior. */
+ to assert it for now to ensure that future code changes do not
+ silently change this behavior. */
gcc_assert (!IS_CMSE_ENTRY (arm_current_func_type ()));
if (arm_current_function_pac_enabled_p ())
- {
- gcc_assert (!(saved_regs_mask & (1 << PC_REGNUM)));
- arm_emit_multi_reg_pop (saved_regs_mask);
- emit_insn (gen_aut_nop ());
- emit_jump_insn (simple_return_rtx);
- }
- else if (num_regs == 1)
- {
- rtx par = gen_rtx_PARALLEL (VOIDmode, rtvec_alloc (2));
- rtx reg = gen_rtx_REG (SImode, PC_REGNUM);
- rtx addr = gen_rtx_MEM (SImode,
- gen_rtx_POST_INC (SImode,
- stack_pointer_rtx));
- set_mem_alias_set (addr, get_frame_alias_set ());
- XVECEXP (par, 0, 0) = ret_rtx;
- XVECEXP (par, 0, 1) = gen_rtx_SET (reg, addr);
- RTX_FRAME_RELATED_P (XVECEXP (par, 0, 1)) = 1;
- emit_jump_insn (par);
- }
+ {
+ gcc_assert (!(saved_regs_mask & (1 << PC_REGNUM)));
+ arm_emit_multi_reg_pop (saved_regs_mask);
+ emit_insn (gen_aut_nop ());
+ emit_jump_insn (simple_return_rtx);
+ }
+ /* Use LDR PC, [sp], #4. Only do this if not optimizing for size and
+ there's a known performance benefit (we don't know this exactly, but
+ preferring LDRD/STRD over LDM/STM is a reasonable proxy). */
+ else if (num_regs == 1
+ && !optimize_size
+ && current_tune->prefer_ldrd_strd)
+ {
+ rtx par = gen_rtx_PARALLEL (VOIDmode, rtvec_alloc (2));
+ rtx reg = gen_rtx_REG (SImode, PC_REGNUM);
+ rtx addr = gen_rtx_MEM (SImode,
+ gen_rtx_POST_INC (SImode,
+ stack_pointer_rtx));
+ set_mem_alias_set (addr, get_frame_alias_set ());
+ XVECEXP (par, 0, 0) = ret_rtx;
+ XVECEXP (par, 0, 1) = gen_rtx_SET (reg, addr);
+ RTX_FRAME_RELATED_P (XVECEXP (par, 0, 1)) = 1;
+ emit_jump_insn (par);
+ }
else
- {
- saved_regs_mask &= ~ (1 << LR_REGNUM);
- saved_regs_mask |= (1 << PC_REGNUM);
- arm_emit_multi_reg_pop (saved_regs_mask);
- }
+ {
+ saved_regs_mask &= ~ (1 << LR_REGNUM);
+ saved_regs_mask |= (1 << PC_REGNUM);
+ arm_emit_multi_reg_pop (saved_regs_mask);
+ }
}
else
{
@@ -28204,7 +28209,10 @@ arm_expand_epilogue (bool really_return)
return_in_pc = true;
}
- if (num_regs == 1 && (!IS_INTERRUPT (func_type) || !return_in_pc))
+ if (num_regs == 1
+ && !optimize_size
+ && current_tune->prefer_ldrd_strd
+ && !(IS_INTERRUPT (func_type) && return_in_pc))
{
for (i = 0; i <= LAST_ARM_REGNUM; i++)
if (saved_regs_mask & (1 << i))
