https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77820
Bug ID: 77820
Summary: A jump threading opportunity with conditionals
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: segher at gcc dot gnu.org
Target Milestone: ---
Consider the following code:
===
void g(void);
void f(long a, long b, long c, long d, int x)
{
int t;
if (x)
t = a < b;
else
t = c < d;
if (t)
g();
}
===
On aarch64 we generate: (it is worse on PowerPC, but this is shorter code)
===
f:
cbz w4, .L2
cmp x0, x1
cset w0, lt
cbnz w0, .L6
.L1:
ret
.p2align 3
.L2:
cmp x2, x3
cset w0, lt
cbz w0, .L1
.L6:
b g
===
while if jump threading was a bit smarter we could generate:
===
f:
cbz w4, .L2
cmp x0, x1
blt .L3
.L1:
ret
.p2align 3
.L2:
cmp x2, x3
bge .L1
.L3:
b g
===
(i.e., do the "if (t)" in both branches of the first "if").
