http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56924
Bug #: 56924
Summary: Folding of checks into a range check should check
upper boundary
Classification: Unclassified
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: [email protected]
ReportedBy: [email protected]
When we are performing folding of checks into a range check, if the values are
at the top-end of the range we should just use a > test instead of normalizing
them into the bottom of the range and using a < test.
For example, consider:
struct stype {
unsigned int pad:4;
unsigned int val:4;
};
void bar (void);
void foo (struct stype input)
{
if ((input.val == 0xe) || (input.val == 0xf))
bar();
}
When compiled at -O2, the original tree generated is:
;; Function foo (null)
;; enabled by -tree-original
{
if (input.val + 2 <= 1)
{
bar ();
}
}
This is likely to be more efficient if we instead generate:
if (input.val >= 0xe)
{
bar ();
}
This can be seen in the inefficient codegen for an ARM cortex-a15:
ubfx r0, r0, #4, #4
add r3, r0, #2
and r3, r3, #15
cmp r3, #1
(the add and the and are not necessary if we change the test condition).
I was able to improve this by adding detection of this case into
build_range_check.
