select: simplify SEL(SEL(x, C, 0), C, 0) --> SEL(x, C, 0) == cond

If the condition of a select is also a select, both with the same
arguments: first a non-zero constant, then a zero constant, then
the outer select is equivalent to the inner one and can thus
be replaced by the result of the inner select (which is its own
condition).

Note: this is a special case of:
	SEL(SEL(x, C, 0), y, z) --> SEL(x, y, z)
      and without this patch we'll have:
	t = SEL(x, C, 0)
	r = SEL(t, C, 0)
      simplified into:
	t = SEL(x, C, 0)	// possibly unused now
	r = SEL(x, C, 0)
      but the present patch do
	t = SEL(x, C, 0)
	r = t
      In other words, functionally, the result is the same but
      now the result is taken from the first instruction.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/simplify.c b/simplify.c
index 6b44d447..1fcfc691 100644
--- a/simplify.c
+++ b/simplify.c
@@ -1791,8 +1791,11 @@ static int simplify_select(struct instruction *insn)
 			// And if that one results in a "zero or not", use the
 			// original conditional instead.
 			//	SEL(SEL(x, C, 0), y, z) --> SEL(x, y, z)
+			//	SEL(SEL(x, C, 0), C, 0) --> SEL(x, C, 0) == cond
 			//	SEL(SEL(x, 0, C), y, z) --> SEL(x, z, y)
 			if (!def->src3->value) {
+				if ((src1 == def->src2) && (src2 == def->src3))
+					return replace_with_pseudo(insn, cond);
 				return replace_pseudo(insn, &insn->cond, def->cond);
 			}
 			if (!def->src2->value) {