Why the code block is not Autovectorized by VC2013 - c

Question

The code block is from mozilla firefox's qcms transform_util.c
void build_output_lut(struct curveType *trc,
uint16_t **output_gamma_lut, size_t *output_gamma_lut_length)
{
if (trc->type == PARAMETRIC_CURVE_TYPE) {
float gamma_table[256];
uint16_t i;
uint16_t *output = malloc(sizeof(uint16_t)*256);
if (!output) {
*output_gamma_lut = NULL;
return;
}
compute_curve_gamma_table_type_parametric(gamma_table, trc->parameter, trc->count);
*output_gamma_lut_length = 256;
for(i = 0; i < 256; i++) {
output[i] = (uint16_t)(gamma_table[i] * 65535);
}
*output_gamma_lut = output;
} else {
if (trc->count == 0) {
*output_gamma_lut = build_linear_table(4096);
*output_gamma_lut_length = 4096;
} else if (trc->count == 1) {
float gamma = 1./u8Fixed8Number_to_float(trc->data[0]);
*output_gamma_lut = build_pow_table(gamma, 4096);
*output_gamma_lut_length = 4096;
} else {
//XXX: the choice of a minimum of 256 here is not backed by any theory,
// measurement or data, however it is what lcms uses.
*output_gamma_lut_length = trc->count;
if (*output_gamma_lut_length < 256)
*output_gamma_lut_length = 256;
*output_gamma_lut = invert_lut(trc->data, trc->count, *output_gamma_lut_length);
}
}
}
For this loop:
for(i = 0; i < 256; i++) {
output[i] = (uint16_t)(gamma_table[i] * 65535);
}
VC2013 will show:
e:\mozilla\hg\nightly\mozilla-central\gfx\qcms\transform_util.c(490) : info C5002: 由于“500”，循环未向量化
MSDN (http://msdn.microsoft.com/en-us/library/jj658585.aspx) shows:
// Code 500 is emitted if the loop has non-vectorizable flow.
// This can include "if", "break", "continue", the conditional
// operator "?", or function calls.
// It also encompasses correct definition and use of the induction
// variable "i", in that the increment "++i" or "i++" must be the last
// statement in the loop.
But the loop above has no if/break/continue, I don't know why it can't be vectorized.

Answer 1

I suspect that this is due to the fact that variable "i" lies in scope of "if"-statement. Thus it does not belong entirely to "for-loop" scope like it would have been in
for(int i = 0; /* etc. */
At such, the compiler's logic would have been like: "I need not only to make gammatable filles with needed values, but to assign same value to "i" as at loop end". Therefore, no vectorization.