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On Wed, Nov 19, 2014 at 10:43 AM, Joey Ye <joey...@arm.com> wrote:
> Current thumb2 -Os generates suboptimal code for following tail call case:
>
> int f4(int b, int a, int c, int d);
> int g(int a, int b, int c, int d)
> { return f4(b, a, c, d); }
>
> arm-none-eabi-gcc -Os -mthumb -mcpu=cortex-m3 test.c
>
> push
> {r4, lr}
> mov r4, r1
> mov r1, r0
> mov r0, r4
> pop {r4, lr}
>
> b f4
>
> There are two issues: The first one is that saving/restoring lr is not
> necessary, as there is no return via pop pc. The second one is that even if
> we managed to avoid lr push/pop, ldmia.w sp!, {r4} is still emitted as there
> is a missing pattern for pop single and code size is not optimal.
>
> This patch fixes these two issues and introduces a shared test case. CSiBE
> thumb2 -Os shows cross board code size reduction, except for one case with 4
> bytes regression. The case is like:
>
> void f ()
> {
> if ()
> ...
> else if ()
> ...
> else g();
> }
>
> There are N=2 non-sibcall returns and S=1 sibcall return. Originally the
> non-sibcall returns are just pop {r4, r5, pc}, now they become
> b.n .Lreturn
>
> .Lreturn:
> pop {r4, r5}
> bx lr
>
> The one byte save from sibcall return does not win the non-sibcall return
> regressions back. In general scenario, number of N non-sibcall returns use
> b.n branching to merged tail, number of S sibcalls save 2 bytes by avoid
> poping lr. It results in 4-2*S bytes regression. In the worst scenario, each
> non-sibcall return has to use b.w branching to merged tail, resulting in
> (N-S)*2 bytes regression. The worst scenario is rare, according to CSiBE.
> The general regression scenario can only regress 2 bytes at most. So I would
> not introduce additional complexity to handle the regression case.
>
> Make check cortex-m3: pass
> thumb2 bootstrap (O2/Os): pass
>
> * config/arm/arm.c (arm_compute_save_reg_mask):
> Do not save lr in case of tail call.
> * config/arm/thumb2.md (*thumb2_pop_single): New pattern.
>
> * gcc.target/arm/thumb2-pop-single.c: New test.
>
> diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
> index 4f04707..20d0b9e 100644
> --- a/gcc/config/arm/arm.c
> +++ b/gcc/config/arm/arm.c
> @@ -19190,6 +19190,7 @@ arm_compute_save_reg_mask (void)
> || (save_reg_mask
> && optimize_size
> && ARM_FUNC_TYPE (func_type) == ARM_FT_NORMAL
> + && !crtl->tail_call_emit
> && !crtl->calls_eh_return))
> save_reg_mask |= 1 << LR_REGNUM;
>
> diff --git a/gcc/config/arm/thumb2.md b/gcc/config/arm/thumb2.md
> index 64acfea..29cfb17 100644
> --- a/gcc/config/arm/thumb2.md
> +++ b/gcc/config/arm/thumb2.md
> @@ -267,6 +267,17 @@
> (set_attr "type" "multiple")]
> )
>
> +;; Pop a single register as its size is preferred over a post-incremental
> load
> +(define_insn "*thumb2_pop_single"
> + [(set (match_operand:SI 0 "low_register_operand" "=r")
> + (mem:SI (post_inc:SI (reg:SI SP_REGNUM))))]
> + "TARGET_THUMB2 && (reload_in_progress || reload_completed)"
> + "pop\t{%0}"
> + [(set_attr "type" "load1")
> + (set_attr "length" "2")
> + (set_attr "predicable" "yes")]
> +)
> +
> ;; We have two alternatives here for memory loads (and similarly for
> stores)
> ;; to reflect the fact that the permissible constant pool ranges differ
> ;; between ldr instructions taking low regs and ldr instructions taking
> high
>
>
>
