[PATCH 2/5] select: simplify SEL(SEL(x, C, 0), y, z) --> SEL(x, y, z) and its dual

[Luc Van Oostenryck <luc.vanoostenryck@xxxxxxxxx>]

If the condition of a select is also a select but with constant
operands, some simplification can be done:
* if the second operand is 0, the original condition can be used,
* idem if the first operand is 0s but the operand must be swapped.

Originally-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@xxxxxxxxx>
---
 simplify.c                                   | 20 ++++++++++++++++++++
 validation/optim/select-select-true-false0.c |  1 -
 validation/optim/select-select-true-false1.c |  1 -
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/simplify.c b/simplify.c
index f2aaa52de84b..6b44d447c0a9 100644
--- a/simplify.c
+++ b/simplify.c
@@ -1748,6 +1748,7 @@ static int simplify_cast(struct instruction *insn)
 static int simplify_select(struct instruction *insn)
 {
 	pseudo_t cond, src1, src2;
+	struct instruction *def;
 
 	cond = insn->src1;
 	src1 = insn->src2;
@@ -1782,6 +1783,25 @@ static int simplify_select(struct instruction *insn)
 		kill_use(&insn->src3);
 		return replace_with_value(insn, 0);
 	}
+
+	switch (DEF_OPCODE(def, cond)) {
+	case OP_SEL:
+		if (constant(def->src2) && constant(def->src3)) {
+			// Is the def of the conditional another select?
+			// 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, 0, C), y, z) --> SEL(x, z, y)
+			if (!def->src3->value) {
+				return replace_pseudo(insn, &insn->cond, def->cond);
+			}
+			if (!def->src2->value) {
+				switch_pseudo(insn, &insn->src2, insn, &insn->src3);
+				return replace_pseudo(insn, &insn->cond, def->cond);
+			}
+		}
+		break;
+	}
 	return 0;
 }
 
diff --git a/validation/optim/select-select-true-false0.c b/validation/optim/select-select-true-false0.c
index 46bd7667400d..4066c2dad6fc 100644
--- a/validation/optim/select-select-true-false0.c
+++ b/validation/optim/select-select-true-false0.c
@@ -4,7 +4,6 @@ int bw(int p, int a, int b) { return ((p ? 0 : 42) ? a : b) == ( p ? b : a); }
 /*
  * check-name: select-select-true-false0
  * check-command: test-linearize -Wno-decl $file
- * check-known-to-fail
  *
  * check-output-ignore
  * check-output-returns: 1
diff --git a/validation/optim/select-select-true-false1.c b/validation/optim/select-select-true-false1.c
index 00ffdcd1bc9b..32c0364de2ba 100644
--- a/validation/optim/select-select-true-false1.c
+++ b/validation/optim/select-select-true-false1.c
@@ -7,7 +7,6 @@ int foo(int p)
 /*
  * check-name: select-select-true-false1
  * check-command: test-linearize -Wno-decl $file
- * check-known-to-fail
  *
  * check-output-ignore
  * check-output-returns: 1
-- 
2.29.2