Current GCC thumb1 has an annoying problem that always assuming far branch.
So it forces to save lr, even when unnecessarily. The most extreme case
complained by partner is:
// compiled with "-mthumb -mcpu=cortex-m0 -Os".
void foo() { for (;;); }
=>
foo:
push {lr} // Crazy!!!
.L2:
b .L2
The reason is that thumb1 far jump is only resolved in the very late pass
"shorten_branch". Prologue/epilogue pass doesn't actually know a branch is
far or not from its attribute. It has to conservatively save/restore lr
whenever there is a branch.
This patch tries to fix it with a simple heuristic, i.e., using function
size to decide if a far jump will likely be used. Function size information
is meaningful in prologue/epilogue pass. The heuristic uses following check
to decide if lr should be saved for far jump:
function_size * 3 >= 2048 // yes: save lr for possible far jump. No: don't
save lr for far jump
The scheme has an issue: if some corner case does break above condition,
there is no chance to fix-up but to ICE. But the heuristic condition is very
conservative. It is base on the worse normal condition that each instruction
is associated with a 4 byte literal ( (2+4)/2=3, blooming size by 3 times ).
I can't think of a real case to trigger the ICE. So I think it should work.
Other approaches than the heuristic scheme are too expensive to implement
for this small size/performance issue. I did explored some but none of them
persuaded myself.
Tests passed:
* build libgcc, libstdc++, newlib, libm
* make check-gcc with cpu=cortex-m0
* Small and extreme test cases
diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 327ef22..ad79451 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -21790,6 +21857,11 @@ thumb1_final_prescan_insn (rtx insn)
else if (conds != CONDS_NOCOND)
cfun->machine->thumb1_cc_insn = NULL_RTX;
}
+
+ /* Check if unexpected far jump is used. */
+ if (cfun->machine->lr_save_eliminated
+ && get_attr_far_jump (insn) == FAR_JUMP_YES)
+ internal_error("Unexpected thumb1 far jump");
}
int
@@ -21815,6 +21887,8 @@ static int
thumb_far_jump_used_p (void)
{
rtx insn;
+ bool far_jump = false;
+ unsigned int func_size = 0;
/* This test is only important for leaf functions. */
/* assert (!leaf_function_p ()); */
@@ -21870,6 +21944,26 @@ thumb_far_jump_used_p (void)
&& get_attr_far_jump (insn) == FAR_JUMP_YES
)
{
+ far_jump = true;
+ }
+ func_size += get_attr_length (insn);
+ }
+
+ /* Attribute far_jump will always be true for thumb1 before
shorten_branch
+ pass. So checking far_jump attribute before shorten_branch isn't much
+ useful.
+
+ Following heuristic tries to estimate more accruately if a far jump
may
+ finally be used. The heuristic is very conservative as there is no
chance
+ to roll-back the decision of not to use far jump.
+
+ Thumb1 long branch offset is -2048 to 2046. The worst case is each
2-byte
+ insn is assiociated with a 4 byte constant pool. Using function size
+ 2048/3 as the threshold is conservative enough. */
+ if (far_jump)
+ {
+ if ((func_size * 3) >= 2048)
+ {
/* Record the fact that we have decided that
the function does use far jumps. */
cfun->machine->far_jump_used = 1;
