From 41a259bd2b1796ddabdae600ee539269a7ddb6a5 Mon Sep 17 00:00:00 2001 From: Peter Lieven Date: Tue, 26 Mar 2013 10:58:32 +0100 Subject: cutils: add a function to find non-zero content in a buffer this adds buffer_find_nonzero_offset() which is a SSE2/Altivec optimized function that searches for non-zero content in a buffer. the function starts full unrolling only after the first few chunks have been checked one by one. analyzing real memory page data has revealed that non-zero pages are non-zero within the first 256-512 bits in most cases. as this function is also heavily used to check for zero memory pages this tweak has been made to avoid the high setup costs of the fully unrolled check for non-zero pages. due to the optimizations used in the function there are restrictions on buffer address and search length. the function can_use_buffer_find_nonzero_content() can be used to check if the function can be used safely. Signed-off-by: Peter Lieven Signed-off-by: Juan Quintela --- util/cutils.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 55 insertions(+) (limited to 'util/cutils.c') diff --git a/util/cutils.c b/util/cutils.c index 1439da4f99..0696a3ba5e 100644 --- a/util/cutils.c +++ b/util/cutils.c @@ -142,6 +142,61 @@ int qemu_fdatasync(int fd) #endif } +/* + * Searches for an area with non-zero content in a buffer + * + * Attention! The len must be a multiple of + * BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR * sizeof(VECTYPE) + * and addr must be a multiple of sizeof(VECTYPE) due to + * restriction of optimizations in this function. + * + * can_use_buffer_find_nonzero_offset() can be used to check + * these requirements. + * + * The return value is the offset of the non-zero area rounded + * down to a multiple of sizeof(VECTYPE) for the first + * BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR chunks and down to + * BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR * sizeof(VECTYPE) + * afterwards. + * + * If the buffer is all zero the return value is equal to len. + */ + +size_t buffer_find_nonzero_offset(const void *buf, size_t len) +{ + const VECTYPE *p = buf; + const VECTYPE zero = (VECTYPE){0}; + size_t i; + + assert(can_use_buffer_find_nonzero_offset(buf, len)); + + if (!len) { + return 0; + } + + for (i = 0; i < BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR; i++) { + if (!ALL_EQ(p[i], zero)) { + return i * sizeof(VECTYPE); + } + } + + for (i = BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR; + i < len / sizeof(VECTYPE); + i += BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR) { + VECTYPE tmp0 = p[i + 0] | p[i + 1]; + VECTYPE tmp1 = p[i + 2] | p[i + 3]; + VECTYPE tmp2 = p[i + 4] | p[i + 5]; + VECTYPE tmp3 = p[i + 6] | p[i + 7]; + VECTYPE tmp01 = tmp0 | tmp1; + VECTYPE tmp23 = tmp2 | tmp3; + if (!ALL_EQ(tmp01 | tmp23, zero)) { + break; + } + } + + return i * sizeof(VECTYPE); +} + /* * Checks if a buffer is all zeroes * -- cgit v1.2.3