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;
}
This patch has been tested on x86_64-pc-linux-gnu with "make bootstrap"
and "make -k check" with no new failures. Ok for mainline?
2021-08-23 Roger Sayle <ro...@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.
Roger
--
diff --git a/gcc/match.pd b/gcc/match.pd
index 0fcfd0e..978a1b0 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. */
/* { 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" } } */
/* { 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" } } */
