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* [gcc r12-3073] Fold sign of LSHIFT_EXPR to eliminate no-op conversions.
@ 2021-08-23 11:38 Roger Sayle
0 siblings, 0 replies; only message in thread
From: Roger Sayle @ 2021-08-23 11:38 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:1d244020246cb155e4de62ca3b302b920a1f513f
commit r12-3073-g1d244020246cb155e4de62ca3b302b920a1f513f
Author: Roger Sayle <roger@nextmovesoftware.com>
Date: Mon Aug 23 12:37:04 2021 +0100
Fold sign of LSHIFT_EXPR to eliminate no-op conversions.
This short patch teaches fold that it is "safe" to change the sign
of a left shift, to reduce the number of type conversions in gimple.
As an example:
unsigned int foo(unsigned int i) {
return (int)i << 8;
}
is currently optimized to:
unsigned int foo (unsigned int i)
{
int i.0_1;
int _2;
unsigned int _4;
<bb 2> [local count: 1073741824]:
i.0_1 = (int) i_3(D);
_2 = i.0_1 << 8;
_4 = (unsigned int) _2;
return _4;
}
with this patch, this now becomes:
unsigned int foo (unsigned int i)
{
unsigned int _2;
<bb 2> [local count: 1073741824]:
_2 = i_1(D) << 8;
return _2;
}
which generates exactly the same assembly language. Aside from the
reduced memory usage, the real benefit is that no-op conversions tend
to interfere with many folding optimizations. For example,
unsigned int bar(unsigned char i) {
return (i ^ (i<<16)) | (i<<8);
}
currently gets (tangled in conversions and) optimized to:
unsigned int bar (unsigned char i)
{
unsigned int _1;
unsigned int _2;
int _3;
int _4;
unsigned int _6;
unsigned int _8;
<bb 2> [local count: 1073741824]:
_1 = (unsigned int) i_5(D);
_2 = _1 * 65537;
_3 = (int) i_5(D);
_4 = _3 << 8;
_8 = (unsigned int) _4;
_6 = _2 | _8;
return _6;
}
but with this patch, bar now optimizes down to:
unsigned int bar(unsigned char i)
{
unsigned int _1;
unsigned int _4;
<bb 2> [local count: 1073741824]:
_1 = (unsigned int) i_3(D);
_4 = _1 * 65793;
return _4;
}
2021-08-23 Roger Sayle <roger@nextmovesoftware.com>
gcc/ChangeLog
* match.pd (shift transformations): Change the sign of an
LSHIFT_EXPR if it reduces the number of explicit conversions.
gcc/testsuite/ChangeLog
* gcc.dg/fold-convlshift-1.c: New test case.
* gcc.dg/fold-convlshift-2.c: New test case.
Diff:
---
gcc/match.pd | 9 +++++++++
gcc/testsuite/gcc.dg/fold-convlshift-1.c | 20 ++++++++++++++++++++
gcc/testsuite/gcc.dg/fold-convlshift-2.c | 20 ++++++++++++++++++++
3 files changed, 49 insertions(+)
diff --git a/gcc/match.pd b/gcc/match.pd
index 0fcfd0ea62c..978a1b0172e 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -3385,6 +3385,15 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
(if (integer_zerop (@2) || integer_all_onesp (@2))
(cmp @0 @2)))))
+/* Both signed and unsigned lshift produce the same result, so use
+ the form that minimizes the number of conversions. */
+(simplify
+ (convert (lshift:s@0 (convert:s@1 @2) INTEGER_CST@3))
+ (if (tree_nop_conversion_p (type, TREE_TYPE (@0))
+ && INTEGRAL_TYPE_P (TREE_TYPE (@2))
+ && TYPE_PRECISION (TREE_TYPE (@2)) <= TYPE_PRECISION (type))
+ (lshift (convert @2) @3)))
+
/* Simplifications of conversions. */
/* Basic strip-useless-type-conversions / strip_nops. */
diff --git a/gcc/testsuite/gcc.dg/fold-convlshift-1.c b/gcc/testsuite/gcc.dg/fold-convlshift-1.c
new file mode 100644
index 00000000000..b6f57f81e72
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/fold-convlshift-1.c
@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+unsigned int foo(unsigned int i)
+{
+ int t1 = i;
+ int t2 = t1 << 8;
+ return t2;
+}
+
+int bar(int i)
+{
+ unsigned int t1 = i;
+ unsigned int t2 = t1 << 8;
+ return t2;
+}
+
+/* { dg-final { scan-tree-dump-not "\\(int\\)" "optimized" } } */
+/* { dg-final { scan-tree-dump-not "\\(unsigned int\\)" "optimized" } } */
+
diff --git a/gcc/testsuite/gcc.dg/fold-convlshift-2.c b/gcc/testsuite/gcc.dg/fold-convlshift-2.c
new file mode 100644
index 00000000000..f21358c4584
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/fold-convlshift-2.c
@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+unsigned int foo(unsigned char c)
+{
+ int t1 = c;
+ int t2 = t1 << 8;
+ return t2;
+}
+
+int bar(unsigned char c)
+{
+ unsigned int t1 = c;
+ unsigned int t2 = t1 << 8;
+ return t2;
+}
+
+/* { dg-final { scan-tree-dump-times "\\(int\\)" 1 "optimized" } } */
+/* { dg-final { scan-tree-dump-times "\\(unsigned int\\)" 1 "optimized" } } */
+
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