Jump threading in tree dom pass prevents if-conversion & following vectorization

["Bingfeng Mei" <bmei at broadcom dot com>]

Hi,
I am doing some investigation on loops can be vectorized
by LLVM, but not GCC. One example is loop that contains
more than one if-else constructs.

typedef signed char int8;
#define FFT         128

typedef struct {
    int8   exp[FFT];
} feq_t;

void test(feq_t *feq)
{
    int k;
    int feqMinimum = 15;
    int8 *exp = feq->exp;

    for (k=0;k<FFT;k++) {
	exp[k] -= feqMinimum;
	if(exp[k]<-15) exp[k] = -15;
	if(exp[k]>15) exp[k]  = 15;
    }
}

Compile it with 4.8.2 on x86_64
~/install-4.8/bin/gcc ghs-algorithms_380.c -O2 -fdump-tree-ifcvt-details -ftree-vectorize  -save-temps

It is not vectorized because if-else constructs are not properly
if-converted. Looking into .ifcvt file, I found the loop is not 
if-converted because of bad if-else structure. One branch jumps directly
into another branch. Digging a bit deeper, I found such structure
is generated by dom1 pass doing jump threading optimization. 
So recompile with 

~/install-4.8/bin/gcc ghs-algorithms_380.c -O2 -fdump-tree-ifcvt-details -ftree-vectorize  -save-temps -fno-tree-dominator-opts

It is magically if-converted and vectorized! Same on our target,
performance is improved greatly in this example.

It seems to me that doing jump threading for architectures
support if-conversion is not a good idea. Original if-else structures 
are damaged so that if-conversion cannot proceed, so are vectorization
and maybe other optimizations. Should we try to identify those "bad" 
jump threading and skip them for such architectures? 

Bingfeng Mei
Broadcom UK