https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96351
Andrew Macleod <amacleod at redhat dot com> changed:
What |Removed |Added
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CC| |amacleod at redhat dot com
--- Comment #2 from Andrew Macleod <amacleod at redhat dot com> ---
I had to do some tweaking and manually inline it, but eventually I managed to
get:
<bb 4> :
pszTmp_11 = str_8(D) + 2;
goto <bb 6>; [INV]
<bb 5> :
pszTmp_13 = pszTmp_5 + 2;
<bb 6> :
# pszTmp_5 = PHI <pszTmp_11(4), pszTmp_13(5)>
_1 = *pszTmp_5;
if (_1 != 0)
goto <bb 5>; [INV]
else
goto <bb 7>; [INV]
<bb 7> :
# pszTmp_16 = PHI <pszTmp_5(6)>
_2 = pszTmp_16 - str_8(D);
_3 = _2 /[ex] 2;
len_12 = (unsigned int) _3;
<bb 8> :
# len_4 = PHI <0(2), len_12(7), 0(3)>
if (len_4 == 0)
goto <bb 9>; [INV]
else
goto <bb 10>; [INV]
Nothing we can do right now. Its possible that the relational work in the next
release *might* be able to sort something out, but thats probably down the
road.
We'll be able to record that
a) pszTmp_11 > str_8(D)
b) pszTmp_13 > pszTmp_5
c) when we see
# pszTmp_5 = PHI <pszTmp_11(4), pszTmp_13(5)>
we might possibly be able to deduce that
d) pszTmp_5 > str_8(D), which then leads to
e) pszTmp_16 > str_8(D).
which means we could know that _2 > 0.
The tricky bit will be knowing that not only is _2 > 0, but its actually [2,
+INF]. Initial plans for relations are not to track possible offsets, but it
could be follow up work.
If we can know that, then we'd be able to calculate len_12 = [1, +INF/2].
With a non-zero range for len_12, then it would be possible for the threader to
create 2 execution paths.. the zero and the non-zero path.
on the non-zero path , there would no longer need to be a reference to len_12,
and dead code could conceivably eliminate the enter else block.
I'd keep this around as a reference, but I don't see this happening any time
soon. If the threaders get a revamp next year... then maybe!
