Hello,
I played some more with odd programs and the effect on control flow
graph construction (as a part of condition coverage support [1]) and
came across this:
int fn (int a, int b, int c) {
int x = 0;
if (a && b) {
if (c) {
goto a_;
} else {
x = a;
}
} else {
a_:
x = (a - b);
}
return x;
}
Run through gcov --conditions I get:
4: 5: if (a && b) {
condition outcomes covered 2/2
condition outcomes covered 2/2
2: 6: if (c) {
condition outcomes covered 2/2
Which is clearly not correct. So I started digging into why and dump the
CFG as the coverage profiling sees it https://i.imgur.com/d0q72rA.png
[2]. I apologize for the labeling, but A2 = a, A3 = b, A5 = c and A9 the
else block. The problem, which is what confuses the algorithm, is that a
and b don't share A9 as a successor (on false) as I would expect.
If I add some operation before the label the problem disappears and a
and b share false-destination again https://i.imgur.com/PSrfaLC.png [3].
} else {
x++;
a_:
x = (a - b);
}
4: 5: if (a && b) {
condition outcomes covered 4/4
2: 6: if (c) {
condition outcomes covered 2/2
When dumping the cfg in the former case with -fdump-tree-cfg-graph I get
a CFG without the split destinations in a and b
https://i.imgur.com/05MCjzp.png [3]. I would assume from this that the
graph dump happens after _more_ CFG transformations than the branch
profiling.
So my questions are:
1. Is the control flow graph expected to be constructed as such where a
and b don't share outcome, or is it to be considered a bug?
2. If yes, would it be problematic to push the branch coverage and
condition profiling to a later stage where the cfg has been fixed?
Thanks,
Jørgen
[1] https://gcc.gnu.org/pipermail/gcc-patches/2022-July/598165.html
[2] dot file:
digraph {
A0 -> A2
A2 -> A3
A2 -> A8
A3 -> A5
A3 -> A4
A8 -> A9
A9 -> A10
A10 -> A11
A11 -> A1
A5 -> A6
A5 -> A7
A4 -> A9
A6 -> A9
A7 -> A10
}
[3] dot file:
digraph {
A0 -> A2
A2 -> A3
A2 -> A7
A3 -> A4
A3 -> A7
A7 -> A8
A8 -> A9
A9 -> A10
A10 -> A1
A4 -> A5
A4 -> A6
A5 -> A8
A6 -> A9
}
[3] dot file:
overlap=false;
subgraph "cluster_fn" {
style="dashed";
color="black";
label="fn ()";
fn_0_basic_block_0
[shape=Mdiamond,style=filled,fillcolor=white,label="ENTRY"];
fn_0_basic_block_1
[shape=Mdiamond,style=filled,fillcolor=white,label="EXIT"];
fn_0_basic_block_2
[shape=record,style=filled,fillcolor=lightgrey,label="{\<bb\ 2\>:\l\
|x\ =\ 0;\l\
|if\ (a\ !=\ 0)\l\
\ \ goto\ \<bb\ 3\>;\ [INV]\l\
else\l\
\ \ goto\ \<bb\ 7\>;\ [INV]\l\
}"];
fn_0_basic_block_3
[shape=record,style=filled,fillcolor=lightgrey,label="{\<bb\ 3\>:\l\
|if\ (b\ !=\ 0)\l\
\ \ goto\ \<bb\ 4\>;\ [INV]\l\
else\l\
\ \ goto\ \<bb\ 7\>;\ [INV]\l\
}"];
fn_0_basic_block_4
[shape=record,style=filled,fillcolor=lightgrey,label="{\<bb\ 4\>:\l\
|if\ (c\ !=\ 0)\l\
\ \ goto\ \<bb\ 5\>;\ [INV]\l\
else\l\
\ \ goto\ \<bb\ 6\>;\ [INV]\l\
}"];
fn_0_basic_block_5
[shape=record,style=filled,fillcolor=lightgrey,label="{\<bb\ 5\>:\l\
|//\ predicted\ unlikely\ by\ goto\ predictor.\l\
goto\ \<bb\ 7\>;\ [INV]\l\
}"];
fn_0_basic_block_6
[shape=record,style=filled,fillcolor=lightgrey,label="{\<bb\ 6\>:\l\
|x\ =\ a;\l\
goto\ \<bb\ 8\>;\ [INV]\l\
}"];
fn_0_basic_block_7
[shape=record,style=filled,fillcolor=lightgrey,label="{\<bb\ 7\>:\l\
|a_:\l\
|x\ =\ a\ -\ b;\l\
}"];
fn_0_basic_block_8
[shape=record,style=filled,fillcolor=lightgrey,label="{\<bb\ 8\>:\l\
|D.2398\ =\ x;\l\
}"];
fn_0_basic_block_9
[shape=record,style=filled,fillcolor=lightgrey,label="{\<bb\ 9\>:\l\
|\<L7\>:\l\
|return\ D.2398;\l\
}"];
fn_0_basic_block_0:s -> fn_0_basic_block_2:n
[style="solid,bold",color=black,weight=100,constraint=true];
fn_0_basic_block_2:s -> fn_0_basic_block_3:n
[style="solid,bold",color=forestgreen,weight=10,constraint=true];
fn_0_basic_block_2:s -> fn_0_basic_block_7:n
[style="solid,bold",color=darkorange,weight=10,constraint=true];
fn_0_basic_block_3:s -> fn_0_basic_block_4:n
[style="solid,bold",color=forestgreen,weight=10,constraint=true];
fn_0_basic_block_3:s -> fn_0_basic_block_7:n
[style="solid,bold",color=darkorange,weight=10,constraint=true];
fn_0_basic_block_4:s -> fn_0_basic_block_5:n
[style="solid,bold",color=forestgreen,weight=10,constraint=true];
fn_0_basic_block_4:s -> fn_0_basic_block_6:n
[style="solid,bold",color=darkorange,weight=10,constraint=true];
fn_0_basic_block_5:s -> fn_0_basic_block_7:n
[style="solid,bold",color=black,weight=100,constraint=true];
fn_0_basic_block_6:s -> fn_0_basic_block_8:n
[style="solid,bold",color=black,weight=100,constraint=true];
fn_0_basic_block_7:s -> fn_0_basic_block_8:n
[style="solid,bold",color=black,weight=100,constraint=true];
fn_0_basic_block_8:s -> fn_0_basic_block_9:n
[style="solid,bold",color=black,weight=100,constraint=true];
fn_0_basic_block_9:s -> fn_0_basic_block_1:n
[style="solid,bold",color=black,weight=10,constraint=true];
fn_0_basic_block_0:s -> fn_0_basic_block_1:n
[style="invis",constraint=true];
}
}
