On Fri, Nov 11, 2016 at 12:10:28PM +0100, Dominik Vogt wrote:
> On Mon, Nov 07, 2016 at 09:29:26PM +0100, Bernd Schmidt wrote:
> > On 10/31/2016 08:56 PM, Dominik Vogt wrote:
> >
> > >combine_simplify_rtx() tries to replace rtx expressions with just two
> > >possible values with an experession that uses if_then_else:
> > >
> > > (if_then_else (condition) (value1) (value2))
> > >
> > >If the original expression is e.g.
> > >
> > > (and (reg) (const_int 2))
> >
> > I'm not convinced that if_then_else_cond is the right place to do
> > this. That function is designed to answer the question of whether an
> > rtx has exactly one of two values and under which condition; I feel
> > it should continue to work this way.
> >
> > Maybe simplify_ternary_expression needs to be taught to deal with this case?
>
> But simplify_ternary_expression isn't called with the following
> test program (only tried it on s390x):
>
> void bar(int, int);
> int foo(int a, int *b)
> {
> if (a)
> bar(0, *b & 2);
> return *b;
> }
>
> combine_simplify_rtx() is called with
>
> (sign_extend:DI (and:SI (reg:SI 61) (const_int 2)))
>
> In the switch it calls simplify_unary_operation(), which return
> NULL. The next thing it does is call if_then_else_cond(), and
> that calls itself with the sign_extend peeled off:
>
> (and:SI (reg:SI 61) (const_int 2))
>
> takes the "BINARY_P (x)" path and returns false. The problem
> exists only if the (and ...) is wrapped in ..._extend, i.e. the
> ondition dealing with (and ...) directly can be removed from the
> patch.
>
> So, all recursive calls to if_then_els_cond() return false, and
> finally the condition in
>
> else if (HWI_COMPUTABLE_MODE_P (mode)
> && pow2p_hwi (nz = nonzero_bits (x, mode))
>
> is true.
>
> Thus, if if_then_else_cond should remain unchanged, the only place
> to fix this would be after the call to if_then_else_cond() in
> combine_simplify_rtx(). Actually, there already is some special
> case handling to override the return code of if_then_else_cond():
>
> cond = if_then_else_cond (x, &true_rtx, &false_rtx);
> if (cond != 0
> /* If everything is a comparison, what we have is highly unlikely
> to be simpler, so don't use it. */
> ---> && ! (COMPARISON_P (x)
> && (COMPARISON_P (true_rtx) || COMPARISON_P (false_rtx))))
> {
> rtx cop1 = const0_rtx;
> enum rtx_code cond_code = simplify_comparison (NE, &cond, &cop1);
>
> ---> if (cond_code == NE && COMPARISON_P (cond))
> return x;
> ...
>
> Should be easy to duplicate the test in the if-body, if that is
> what you prefer:
>
> ...
> if (HWI_COMPUTABLE_MODE_P (GET_MODE (x))
> && pow2p_hwi (nz = nonzero_bits (x, GET_MODE (x)))
> && ! ((code == SIGN_EXTEND || code == ZERO_EXTEND)
> && GET_CODE (XEXP (x, 0)) == AND
> && CONST_INT_P (XEXP (XEXP (x, 0), 0))
> && UINTVAL (XEXP (XEXP (x, 0), 0)) == nz))
> return x;
>
> (untested)
Updated and tested version of the patch attached. The extra logic
is now in combine_simplify_rtx.
Ciao
Dominik ^_^ ^_^
--
Dominik Vogt
IBM Germany
gcc/ChangeLog
* combine.c (combine_simplify_rtx): Suppress replacement of
"(and (reg) (const_int bit))" with "if_then_else".
>From 2ebe692928b4ebee3fa6dc02136980801a04b33d Mon Sep 17 00:00:00 2001
From: Dominik Vogt <v...@linux.vnet.ibm.com>
Date: Mon, 31 Oct 2016 09:00:31 +0100
Subject: [PATCH] Do not simplify "(and (reg) (const bit)" to if_then_else.
combine_simplify_rtx() tries to replace rtx expressions with just two
possible values with an experession that uses if_then_else:
(if_then_else (condition) (value1) (value2))
If the original expression is e.g.
(and (reg) (const_int 2))
where the constant is the mask for a single bit, the replacement results
in a more complex expression than before:
(if_then_else (ne (zero_extract (reg) (1) (31))) (2) (0))
Similar replacements are done for
(signextend (and ...))
(zeroextend (and ...))
Suppress the replacement this special case in if_then_else_cond().
---
gcc/combine.c | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/gcc/combine.c b/gcc/combine.c
index b22a274..457fe8a 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -5575,10 +5575,23 @@ combine_simplify_rtx (rtx x, machine_mode op0_mode, int
in_dest,
{
rtx cop1 = const0_rtx;
enum rtx_code cond_code = simplify_comparison (NE, &cond, &cop1);
+ unsigned HOST_WIDE_INT nz;
if (cond_code == NE && COMPARISON_P (cond))
return x;
+ /* If the operation is an AND wrapped in a SIGN_EXTEND or ZERO_EXTEND
+ with either operand being just a constant single bit value, do
+ nothing since IF_THEN_ELSE is likely to increase the expression's
+ complexity. */
+ if (HWI_COMPUTABLE_MODE_P (mode)
+ && pow2p_hwi (nz = nonzero_bits (x, mode))
+ && ! ((code == SIGN_EXTEND || code == ZERO_EXTEND)
+ && GET_CODE (XEXP (x, 0)) == AND
+ && CONST_INT_P (XEXP (XEXP (x, 0), 0))
+ && UINTVAL (XEXP (XEXP (x, 0), 0)) == nz))
+ return x;
+
/* Simplify the alternative arms; this may collapse the true and
false arms to store-flag values. Be careful to use copy_rtx
here since true_rtx or false_rtx might share RTL with x as a
--
2.3.0
