[RFC] Remove PUSH_ARGS_REVERSED from the RTL expander.

[James Greenhalgh]

Hi,

Consider this testcase:

  extern void foo (int a, int b, int c, int d);

  void
  bar (int b, int c, int d)
  {
      foo (3, b, c, d);
  }

For many ABI's we will have on entry to the function

  r0 = b
  r1 = c
  r2 = d

If we process arguments to the call to foo left-to-right
(PUSH_ARGS_REVERSED == 0) we assign temporaries, and then hard registers
in this order:

  t0 = 3
  t1 = r0
  t2 = r1
  t3 = r2

  r0 = t0
  r1 = t1
  r2 = t2
  r3 = t3

We can't eliminate any of these temporaries as their live ranges overlap.

However, If we process the arguments right-to-left (PUSH_ARGS_REVERSED == 1),
we get this order:

  t0 = 3
  t1 = r0
  t2 = r1
  t3 = r2

  r3 = t3
  r2 = t2
  r1 = t1
  r0 = t0

And then we can start eliminating temporaries we don't need and get:

  r3 = r2
  r2 = r1
  r1 = r0
  r0 = 3

Which is much neater.

It seems to me that this would probably be of benefit on all targets.

This would be an argument for setting PUSH_ARGS_REVERSED to 1 by default
for all targets. However, this would have a perceivable impact on argument
evaluation order (which is unspecified in C, but people have shown
sensitivity to it changing in the past and we suspect some people may
have built systems relying on the behviour... ho hum...).

However, now that all side effects are handled when we are in SSA
form, it should be possible to refactor calls.c and expr.c as if
PUSH_ARGS_REVERSED were always defined to 1, without changing the
argument evaluation order in gimplify.c.

In this way, we have the best of both worlds; we maintain argument
evaluation order and gain the optimizations given by removing
overlapping live ranges for parameters.

I've bootstrapped and regression tested this on x86, ARM and AArch64 - but
I am well aware these are fairly standard targets, do you have any
suggestions or comments on which targets this change will affect?

If not, is this OK for stage-1?

Thanks,
James

---
gcc/

2014-03-21  James Greenhalgh  <james.greenhalgh@arm.com>

	* calls.c (initialize_argument_information): Always treat
	PUSH_ARGS_REVERSED as 1, simplify code accordingly.
	(expand_call): Likewise.
	(emit_library_call_calue_1): Likewise.
	* expr.c (PUSH_ARGS_REVERSED): Do not define.
	(emit_push_insn): Always treat PUSH_ARGS_REVERSED as 1, simplify
	code accordingly.
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