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From: "rsandifo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/97588] New: Overzealous SRA of boolean bitfields
Date: Tue, 27 Oct 2020 08:43:55 +0000 [thread overview]
Message-ID: <bug-97588-4@http.gcc.gnu.org/bugzilla/> (raw)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97588
Bug ID: 97588
Summary: Overzealous SRA of boolean bitfields
Product: gcc
Version: 10.2.1
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: rsandifo at gcc dot gnu.org
Target Milestone: ---
For the nonsense code (reduced from real code):
--------------------------------------------------
struct s
{
unsigned int foo : 11;
unsigned int flag1 : 1;
unsigned int bar : 11;
unsigned int flag2 : 1;
};
void
f (int n, int *x, struct s *ptr, struct s flags)
{
for (int i = 0; i < n; ++i)
if (x[i] == 1)
flags.flag1 = 1;
else if (x[i] == 2)
flags.flag2 = 1;
else if (x[i] == 3)
{
if (flags.flag1)
*ptr++ = flags;
}
else if (x[i] == 4)
{
if (flags.flag2)
*ptr++ = flags;
}
else
*ptr++ = flags;
}
--------------------------------------------------
SRA significantly pessimises the output. At the machine level,
each update to flags is usually a simple register OR, bit-test,
or move, but SRA instead decides to split flags up into 4
pieces and reassemble it for "*ptr++ = flags" (which in the
original code is the hot statement).
next reply other threads:[~2020-10-27 8:43 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-27 8:43 rsandifo at gcc dot gnu.org [this message]
2020-10-27 9:05 ` [Bug tree-optimization/97588] " rguenth at gcc dot gnu.org
2021-01-22 18:28 ` jamborm at gcc dot gnu.org
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