From: Philipp Tomsich <p...@gnu.org>
csmith managed to sneak a shift wider than the bit-width of a register
past the frontend (found when addressing a bug in our bitmanip machine
description): no warning is given and an unneeded shift is generated.
This behaviour was validated for the resulting assembly both for RISC-V
and AArch64.
This matches (x << C), where C is contant and C > precicison(x), and
rewrites it to (const_int 0). This has been confirmed to remove the
redundant shift instruction both for AArch64 and RISC-V.
---
gcc/match.pd | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/gcc/match.pd b/gcc/match.pd
index 349eab6..2309175 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -764,6 +764,12 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
(cabss (ops @0))
(cabss @0))))
+/* Fold (x << C), where C > precision(type) into 0. */
+(simplify
+ (lshift @0 INTEGER_CST@1)
+ (if (wi::ltu_p (TYPE_PRECISION (TREE_TYPE (@0)), wi::to_wide(@1)))
+ { build_zero_cst (TREE_TYPE (@0)); } ))
+
/* Fold (a * (1 << b)) into (a << b) */
(simplify
(mult:c @0 (convert? (lshift integer_onep@1 @2)))
--
1.8.3.1
