From c41492423140e1573df68d1c98e825ae7593741f Mon Sep 17 00:00:00 2001
From: Roger Sayle <roger@nextmovesoftware.com>
Date: Wed, 11 Oct 2023 08:08:04 +0100
Subject: [PATCH] Optimize (ne:SI (subreg:QI (ashift:SI x 7) 0) 0) as (and:SI x
1).
This patch is the middle-end piece of an improvement to PRs 101955 and
106245, that adds a missing simplification to the RTL optimizers.
This transformation is to simplify (char)(x << 7) != 0 as x & 1.
Technically, the cast can be any truncation, where shift is by one
less than the narrower type's precision, setting the most significant
(only) bit from the least significant bit.
This transformation applies to any target, but it's easy to see
(and add a new test case) on x86, where the following function:
int f(int a) { return (a << 31) >> 31; }
currently gets compiled with -O2 to:
foo: movl %edi, %eax
sall $7, %eax
sarb $7, %al
movsbl %al, %eax
ret
but with this patch, we now generate the slightly simpler.
foo: movl %edi, %eax
sall $31, %eax
sarl $31, %eax
ret
2023-10-11 Roger Sayle <roger@nextmovesoftware.com>
gcc/ChangeLog
PR middle-end/101955
PR tree-optimization/106245
* simplify-rtx.cc (simplify_relational_operation_1): Simplify
the RTL (ne:SI (subreg:QI (ashift:SI x 7) 0) 0) to (and:SI x 1).
gcc/testsuite/ChangeLog
* gcc.target/i386/pr106245-1.c: New test case.
---
gcc/simplify-rtx.cc | 17 +++++++++++++++++
gcc/testsuite/gcc.target/i386/pr106245-1.c | 10 ++++++++++
2 files changed, 27 insertions(+)
create mode 100644 gcc/testsuite/gcc.target/i386/pr106245-1.c
diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
index bd9443dbcc28..69d87579d9c8 100644
--- a/gcc/simplify-rtx.cc
+++ b/gcc/simplify-rtx.cc
@@ -6109,6 +6109,23 @@ simplify_context::simplify_relational_operation_1 (rtx_code code,
break;
}
+ /* (ne:SI (subreg:QI (ashift:SI x 7) 0) 0) -> (and:SI x 1). */
+ if (code == NE
+ && op1 == const0_rtx
+ && (op0code == TRUNCATE
+ || (partial_subreg_p (op0)
+ && subreg_lowpart_p (op0)))
+ && SCALAR_INT_MODE_P (mode)
+ && STORE_FLAG_VALUE == 1)
+ {
+ rtx tmp = XEXP (op0, 0);
+ if (GET_CODE (tmp) == ASHIFT
+ && GET_MODE (tmp) == mode
+ && CONST_INT_P (XEXP (tmp, 1))
+ && is_int_mode (GET_MODE (op0), &int_mode)
+ && INTVAL (XEXP (tmp, 1)) == GET_MODE_PRECISION (int_mode) - 1)
+ return simplify_gen_binary (AND, mode, XEXP (tmp, 0), const1_rtx);
+ }
return NULL_RTX;
}
diff --git a/gcc/testsuite/gcc.target/i386/pr106245-1.c b/gcc/testsuite/gcc.target/i386/pr106245-1.c
new file mode 100644
index 000000000000..a0403e931b34
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr106245-1.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+int f(int a)
+{
+ return (a << 31) >> 31;
+}
+
+/* { dg-final { scan-assembler-not "sarb" } } */
+/* { dg-final { scan-assembler-not "movsbl" } } */
--
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