[APInt] improve initialization performance (#106945)

The purpose is to save an extra memset in both cases:

1. When `int64_t(val) < 0`, zeroing out is redundant as the subsequent
for-loop will initialize to `val .. 0xFFFFF ....`. Instead we should
only create an uninitialized buffer, and transform the slow for-loop
into a memset to initialize the higher words to `0xFF`.
2. In the other case, first we create an uninitialized array (`new
int64_t[]`) and _then_ we zero it out with `memset`. But this can be
combined in one operation with `new int64_t[]()`, which
default-initializes the array.

On one example where use of APInt was heavy, this improved compile time
by 1%.

0 files changed, 0 insertions, 0 deletions