[RFC PATCH] ira: accept loads via argp rtx in validate_equiv_mem

[Alexander Monakov <amonakov@ispras.ru>]

With this patch at hand, I'd like to discuss a code generation problem, which
my patch solves only partially.  FWIW, it passes bootstrap/regtest on x86-64.

With other patches in series applied, GCC with -fno-plt can generate tail
calls in PIC mode more frequently, but sometimes poorer code is generated.
I've tried to look for possible causes, and found one issue so far.

Consider the following testcase:

void foo1(int a, int b, int c, int d, int e, int f, int g, int h);
int bar(int x);
void foo2(int a, int b, int c, int d, int e, int f, int g, int h)
{
  bar(a);
  foo1(a, b, c, d, e, f, g, h);
}

Comparing x86 code generation with -O2 -m32 and with/without -fPIC, you can
see that -fPIC happens to produce smaller code.  Without -fPIC, GCC
saves/restores all arguments before/after call to 'bar'.

The reason for that is without -fPIC, GCC performs tail call optimization on
'foo1', and that causes it to drop REG_EQUIV notes for incoming arguments in
fixup_tail_calls.  After that, code generation diverges at IRA stage, where
lack of equivalences prevents loads of pseudos to be moved to the point of
first use.

The patch tries to repair the problem by allowing REG_EQUIV notes to be
resynthesized at ira init for loads that happen via `argp' rtx.  It helps for
the simple testcase above, but not for problematic Clang/LLVM functions where
I noticed the issue.

I hope there's a way around the 'big hammer' approach of fixup_tail_calls.
Might it be possible instead of dropping REG_EQUIV notes, to copy incoming
arguments into other pseudos just prior to stack pointer adjustment in
preparation for tailcall?

diff --git a/gcc/ira.c b/gcc/ira.c
index ea2b69f..e6b82e2 100644
--- a/gcc/ira.c
+++ b/gcc/ira.c
@@ -3001,13 +3001,16 @@ validate_equiv_mem (rtx_insn *start, rtx reg, rtx memref)
 
       /* This used to ignore readonly memory and const/pure calls.  The problem
 	 is the equivalent form may reference a pseudo which gets assigned a
 	 call clobbered hard reg.  When we later replace REG with its
 	 equivalent form, the value in the call-clobbered reg has been
 	 changed and all hell breaks loose.  */
-      if (CALL_P (insn))
+      rtx addr = XEXP (memref, 0);
+      if (GET_CODE (addr) == PLUS && GET_CODE (XEXP (addr, 1)) == CONST_INT)
+	addr = XEXP (addr, 0);
+      if (CALL_P (insn) && addr != arg_pointer_rtx)
 	return 0;
 
       note_stores (PATTERN (insn), validate_equiv_mem_from_store, NULL);
 
       /* If a register mentioned in MEMREF is modified via an
 	 auto-increment, we lose the equivalence.  Do the same if one