After an OP_ZEXT, it is guaranted that the sign bit is zero.
So, an arithmetic right-shift following an OP_ZEXT gives the
same result as an logical right-shift which is simpler to handle
and further optimize.
So, when preceded by an OP_ZEXT, replace OP_ASR by OP_LSR.
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@xxxxxxxxx>
---
simplify.c | 12 ++++++++++++
validation/optim/zext-asr.c | 13 +++++++++++++
2 files changed, 25 insertions(+)
create mode 100644 validation/optim/zext-asr.c
diff --git a/simplify.c b/simplify.c
index 65ea87d94..513652d2b 100644
--- a/simplify.c
+++ b/simplify.c
@@ -549,6 +549,7 @@ undef:
static int simplify_shift(struct instruction *insn, pseudo_t pseudo, long long value)
{
+ struct instruction *def;
unsigned int size;
if (!value)
@@ -562,6 +563,17 @@ static int simplify_shift(struct instruction *insn, pseudo_t pseudo, long long v
}
switch (insn->opcode) {
case OP_ASR:
+ switch(DEF_OPCODE(def, pseudo)) {
+ case OP_ZEXT:
+ // transform:
+ // zext.N %t <- (O) %a
+ // asr.N %r <- %t, C
+ // into
+ // zext.N %t <- (O) %a
+ // lsr.N %r <- %t, C
+ insn->opcode = OP_LSR;
+ return REPEAT_CSE;
+ }
break;
case OP_LSR:
size = operand_size(insn, pseudo);
diff --git a/validation/optim/zext-asr.c b/validation/optim/zext-asr.c
new file mode 100644
index 000000000..5f235ad80
--- /dev/null
+++ b/validation/optim/zext-asr.c
@@ -0,0 +1,13 @@
+unsigned short foo(unsigned short a)
+{
+ return a >> 16;
+}
+
+/*
+ * check-name: zext-asr
+ * check-command: test-linearize -Wno-decl $file
+ *
+ * check-output-ignore
+ * check-output-contains: ret\\..*\\$0
+ * check-output-excludes: asr\\.
+ */
--
2.18.0
--
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