Andreas Schwab and Gunther Nikl have pointed out that GCC
will incorrectly create "on purpose" impossible branch instructions.
Reason Summary:
- GCC is able to simplify certain compares.
- GCC seems to be unable to correctly rewrite the corresponding branch
instructions.
- GCC thereby generates branches that are by definition impossible to do.
Example:
void foo (unsigned long j)
{
unsigned int i;
for (i = 0; i < (j>>5); ++i)
;
}
In this example the generated code does include a compare with a variable that
is ZERO.
GCC correctly knows that it can simplify a compare against ZERO.
But GCC fails to correctly adapt the condition codes.
Background:
A compare of two unsigned variables can set the CARRY flag.
A compare against ZERO can never set the CARRY flag.
While GCC recognizes that it can rewrite a CMP with 0 ,
GCC does not rewrite the branch that checks for the Carry.
GCC hereby creates branches that include conditions that are known to be
impossible at compile time.
> The following link should be the thread about this issue
> http://gcc.gnu.org/ml/gcc/2003-10/msg01236.html
The problem that we describe here leads to the following:
GCC creates branches that include impossible conditions.
GCC does not remove the impossible conditions.
GCC wants to rewrite the CMP with 0 with a more efficient code.
But the 68K backend need to forbid this.
Normally it would be possible to leave the CMP 0,variable simply away.
And all branches that test for a condition that could actually be set by a cmp
with 0 would be possible to evaluate.
But as GCC writes branches which include known impossible conditions leaving
the cmp away is not possible.
The CMP with 0 is explicitly needed to ensure that all flags are cleared to
ensure that the branches to known impossible conditions are not taken.
It would be great if you could fix the reason for this instead curing the
sympton.
As this would allow the backend to really remove the unneeded CMP instructions
thereby generating smaller and faster code.
Many thanks in advance
Gunnar von Boehn
--
Summary: GCC generates impossible BRANCH instruction
Product: gcc
Version: 4.4.0
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: gunnar at greyhound-data dot com
GCC build triplet: m68k-linux-gnu
GCC host triplet: m68k-linux-gnu
GCC target triplet: m68k-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36772
