https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107443
Bug ID: 107443
Summary: Switch conversion removing code
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: amacleod at redhat dot com
Target Milestone: ---
Compile the following with -O2 -fdump-tree-all -fdisable-tree-evrp
void dead (unsigned n);
void foo (unsigned n);
void func (unsigned n)
{
if (n == 0)
__builtin_unreachable();
if (n == 1)
__builtin_unreachable();
if (n == 2)
__builtin_unreachable();
if (n == 3)
__builtin_unreachable();
if (n == 4)
__builtin_unreachable();
if (n == 5)
__builtin_unreachable();
if (n == 6)
__builtin_unreachable();
if (n == 7)
__builtin_unreachable();
foo (n);
if (n < 8)
dead (n);
}
iftoswitch converts the series of if's into a switch:
<bb 2> :
switch (n_2(D)) <default: <L26> [INV], case 0: <L18> [INV], case 1: <L19>
[INV], case 2: <L20> [INV], case 3: <L21> [INV], case 4: <L22> [INV], case 5:
<L23> [INV], case 6: <L24> [INV], case 7: <L25> [INV]>
<bb 3> :
<L18>:
__builtin_unreachable ();
<bb 4> :
<L19>:
__builtin_unreachable ();
<bb 5> :
<L20>:
__builtin_unreachable ();
<bb 6> :
<L21>:
__builtin_unreachable ();
<bb 7> :
<L22>:
__builtin_unreachable ();
<bb 8> :
<L23>:
__builtin_unreachable ();
<bb 9> :
<L24>:
__builtin_unreachable ();
<bb 10> :
<L25>:
__builtin_unreachable ();
<bb 11> :
<L26>:
foo (n_2(D));
if (n_2(D) <= 7)
goto <bb 12>; [INV]
else
goto <bb 13>; [INV]
<bb 12> :
dead (n_2(D));
switch conversion then runs, eliminates all the blocks, then bails on the
conversion, leaving the IL without any of the __builtin_unreachable calls:
Beginning to process the following SWITCH statement ((null):0) : -------
switch (n_2(D)) <default: <L26> [INV], case 0: <L18> [INV], case 1: <L19>
[INV], case 2: <L20> [INV], case 3: <L21> [INV], case 4: <L22> [INV], case 5:
<L23> [INV], case 6: <L24> [INV], case 7: <L25> [INV]>
Removing basic block 3
Removing basic block 4
Removing basic block 5
Removing basic block 6
Removing basic block 7
Removing basic block 8
Removing basic block 9
Removing basic block 10
Bailing out - switch is a degenerate case
--------------------------------
Merging blocks 2 and 11
void func (unsigned int n)
{
<bb 2> :
foo (n_2(D));
if (n_2(D) <= 7)
goto <bb 3>; [INV]
else
goto <bb 4>; [INV]
<bb 3> :
dead (n_2(D));
<bb 4> :
return;
}
And now the optimizers cannot remove the call to dead() because we lose all the
range information inferred from the __builtin_unreachable calls.
