[PATCH v3 7/8] transform (A << S) >> S into A & (-1 >> S)

[Luc Van Oostenryck <luc.vanoostenryck@gmail.com>]

More exactly, transform it into:
	A & (Mask(size) >> S)
which is equivalent to:
	A & Mask(size - S)
where Mask(X) is ((1 << X) - 1)

This transformation is especially usefull when simplifying code
accessing bitfields or for other masking manipulations.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
---
 simplify.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/simplify.c b/simplify.c
index f1a898700..23cdd19ae 100644
--- a/simplify.c
+++ b/simplify.c
@@ -416,6 +416,8 @@ static int simplify_lsr(struct instruction *insn, pseudo_t pseudo, long long val
 {
 	struct instruction *def;
 	unsigned long long mask;
+	unsigned int width;
+	pseudo_t old;
 
 	if (!value)
 		return replace_with_pseudo(insn, pseudo);
@@ -434,6 +436,21 @@ static int simplify_lsr(struct instruction *insn, pseudo_t pseudo, long long val
 		insn->opcode = OP_AND;
 		insn->src2 = value_pseudo(mask >> value);
 		return REPEAT_CSE;
+	case OP_SHL:
+		// replace (A << S) >> S
+		// by      A & (Mask(size) >> S)
+		def = insn->src1->def;
+		if (!constant(def->src2))
+			break;
+		if (def->src2->value != value)
+			break;
+		width = insn->size - value;
+		insn->src2 = value_pseudo((1ULL << width) - 1);
+		insn->opcode = OP_AND;
+		old = insn->src1;
+		use_pseudo(insn, def->src1, &insn->src1);
+		remove_usage(old, &insn->src1);
+		return REPEAT_CSE;
 	}
 	return 0;
 }
-- 
2.13.2