https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105030
Bug ID: 105030
Summary: store motion if-change flag causes if-conversion
optimization can't be taken.
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: guihaoc at gcc dot gnu.org
Target Milestone: ---
// source code
extern void bar (double *, int);
void foo (double a[], int n)
{
double atemp = 0.5;
for (int i = 0; i < n; i++)
if (a[i] < atemp)
atemp = a[i];
bar (&atemp, n);
}
// -O3 -fdump-tree-lim2
if (_4 < atemp.0_5)
goto <bb 4>; [50.00%]
else
goto <bb 5>; [50.00%]
<bb 4> [local count: 477815112]:
atemp_lsm.4_24 = _4;
atemp_lsm_flag.5_25 = 1;
It creates the lsm flag in lim2 pass. So the "then" block has two sets which
blocks the if-conversion optimization.
//assemble -O3 -ffast-math -fno-unroll-loops on ppc64le
.L5:
lfd 0,0(3)
addi 3,3,8
fcmpu 0,12,0
ble 0,.L3
fmr 12,0
li 9,1
.L3:
bdnz .L5
andi. 9,9,0x1
beq 0,.L2
stfd 12,32(1)
Inefficient fcmpu is used. If the source code is tweaked as below, the
efficient xvmindp is generated.
// tweaked source code
extern void bar (double *, int);
void foo (double a[], int n)
{
double atemp = 0.5;
for (int i = 0; i < n; i++)
if (a[i] < atemp)
atemp = a[i];
double btemp = atemp;
bar (&btemp, n);
}
//assembly
.L4:
lxv 0,0(9)
addi 9,9,16
xvmindp 12,12,0
bdnz .L4
