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8078497: C2's superword optimization causes unaligned memory accesses

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Prevent vectorization of memory operations with different invariant offsets if unaligned memory accesses are not allowed.

Reviewed-by: kvn
This commit is contained in:
Tobias Hartmann 2015-05-08 12:19:17 +02:00
parent 58a1361125
commit 43a5abc876
3 changed files with 204 additions and 21 deletions
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77
hotspot/src/share/vm/opto/superword.cpp
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@ -293,6 +293,13 @@ void SuperWord::find_adjacent_refs() {
// if unaligned memory access is not allowed because number of
// iterations in pre-loop will be not enough to align it.
create_pack = false;
} else {
SWPointer p2(best_align_to_mem_ref, this);
if (align_to_ref_p.invar() != p2.invar()) {
// Do not vectorize memory accesses with different invariants
// if unaligned memory accesses are not allowed.
create_pack = false;
}
}
}
} else {
@ -516,24 +523,50 @@ bool SuperWord::ref_is_alignable(SWPointer& p) {
if (ABS(span) == mem_size && (ABS(offset) % mem_size) == 0) {
return true;
}
// If initial offset from start of object is computable,
// compute alignment within the vector.
// If the initial offset from start of the object is computable,
// check if the pre-loop can align the final offset accordingly.
//
// In other words: Can we find an i such that the offset
// after i pre-loop iterations is aligned to vw?
// (init_offset + pre_loop) % vw == 0 (1)
// where
// pre_loop = i * span
// is the number of bytes added to the offset by i pre-loop iterations.
//
// For this to hold we need pre_loop to increase init_offset by
// pre_loop = vw - (init_offset % vw)
//
// This is only possible if pre_loop is divisible by span because each
// pre-loop iteration increases the initial offset by 'span' bytes:
// (vw - (init_offset % vw)) % span == 0
//
int vw = vector_width_in_bytes(p.mem());
assert(vw > 1, "sanity");
if (vw % span == 0) {
Node* init_nd = pre_end->init_trip();
if (init_nd->is_Con() && p.invar() == NULL) {
int init = init_nd->bottom_type()->is_int()->get_con();
int init_offset = init * p.scale_in_bytes() + offset;
assert(init_offset >= 0, "positive offset from object start");
Node* init_nd = pre_end->init_trip();
if (init_nd->is_Con() && p.invar() == NULL) {
int init = init_nd->bottom_type()->is_int()->get_con();
int init_offset = init * p.scale_in_bytes() + offset;
assert(init_offset >= 0, "positive offset from object start");
if (vw % span == 0) {
// If vm is a multiple of span, we use formula (1).
if (span > 0) {
return (vw - (init_offset % vw)) % span == 0;
} else {
assert(span < 0, "nonzero stride * scale");
return (init_offset % vw) % -span == 0;
}
} else if (span % vw == 0) {
// If span is a multiple of vw, we can simplify formula (1) to:
// (init_offset + i * span) % vw == 0
// =>
// (init_offset % vw) + ((i * span) % vw) == 0
// =>
// init_offset % vw == 0
//
// Because we add a multiple of vw to the initial offset, the final
// offset is a multiple of vw if and only if init_offset is a multiple.
//
return (init_offset % vw) == 0;
}
}
return false;
@ -545,17 +578,23 @@ int SuperWord::get_iv_adjustment(MemNode* mem_ref) {
SWPointer align_to_ref_p(mem_ref, this);
int offset = align_to_ref_p.offset_in_bytes();
int scale = align_to_ref_p.scale_in_bytes();
int elt_size = align_to_ref_p.memory_size();
int vw = vector_width_in_bytes(mem_ref);
assert(vw > 1, "sanity");
int stride_sign = (scale * iv_stride()) > 0 ? 1 : -1;
// At least one iteration is executed in pre-loop by default. As result
// several iterations are needed to align memory operations in main-loop even
// if offset is 0.
int iv_adjustment_in_bytes = (stride_sign * vw - (offset % vw));
int elt_size = align_to_ref_p.memory_size();
assert(((ABS(iv_adjustment_in_bytes) % elt_size) == 0),
err_msg_res("(%d) should be divisible by (%d)", iv_adjustment_in_bytes, elt_size));
int iv_adjustment = iv_adjustment_in_bytes/elt_size;
int iv_adjustment;
if (scale != 0) {
int stride_sign = (scale * iv_stride()) > 0 ? 1 : -1;
// At least one iteration is executed in pre-loop by default. As result
// several iterations are needed to align memory operations in main-loop even
// if offset is 0.
int iv_adjustment_in_bytes = (stride_sign * vw - (offset % vw));
assert(((ABS(iv_adjustment_in_bytes) % elt_size) == 0),
err_msg_res("(%d) should be divisible by (%d)", iv_adjustment_in_bytes, elt_size));
iv_adjustment = iv_adjustment_in_bytes/elt_size;
} else {
// This memory op is not dependent on iv (scale == 0)
iv_adjustment = 0;
}
#ifndef PRODUCT
if (TraceSuperWord)