sizehint! now supports shrinking arrays JuliaLang#2879 (JuliaLang#26201)

src/array.c

@@ -875,6 +875,29 @@ JL_DLLEXPORT void jl_array_grow_beg(jl_array_t *a, size_t inc)
  jl_array_grow_at_beg(a, 0, inc, n);
 }
 
+STATIC_INLINE void jl_array_shrink(jl_array_t *a, size_t dec)
+{
+ //if we dont manage this array return
+ if (a->flags.how == 0) return;
+
+ int newbytes = (a->maxsize - dec) * a->elsize;
+ int oldnbytes = (a->maxsize) * a->elsize;
+
+ char *originalptr = ((char*) a->data) - a->offset;
+ if (a->flags.how == 1) {
+ //this is a julia-allocated buffer that needs to be marked
+ }
+ else if (a->flags.how == 2) {
+ //malloc-allocated pointer this array object manages
+ a->data = jl_gc_managed_realloc(originalptr, newbytes, oldnbytes,
+ a->flags.isaligned, (jl_value_t*) a);
+ a->maxsize -= dec;
+ }
+ else if (a->flags.how == 3) {
+ //this has has a pointer to the object that owns the data
+ }
+}
+
 static size_t jl_array_limit_offset(jl_array_t *a, size_t offset)
 {
  // make sure offset doesn't grow forever due to deleting at beginning
@@ -1009,14 +1032,25 @@ JL_DLLEXPORT void jl_array_del_end(jl_array_t *a, size_t dec)
 JL_DLLEXPORT void jl_array_sizehint(jl_array_t *a, size_t sz)
 {
  size_t n = jl_array_nrows(a);
- if (sz <= n)
- return;
- size_t inc = sz - n;
- jl_array_grow_end(a, inc);
- a->nrows = n;
+
+ int min = a->offset + a->length;
+ sz = (sz < min) ? min : sz;
+
+ if (sz <= a->maxsize) {
+ size_t dec = a->maxsize - sz;
+ //if we dont save at least an eighth of maxsize then its not worth it to shrink
+ if (dec < a->maxsize / 8) return;
+ jl_array_shrink(a, dec);
+ }
+ else {
+ size_t inc = sz - n;
+ jl_array_grow_end(a, inc);
+
+ a->nrows = n;
 #ifdef STORE_ARRAY_LEN
- a->length = n;
+  a->length = n;
 #endif
+ }
 }
 
 JL_DLLEXPORT jl_array_t *jl_array_copy(jl_array_t *ary)

test/arrayops.jl

@@ -145,6 +145,28 @@ end
  @test isa(reshape(s, Val(N)), Base.ReshapedArray{Int,N})
  end
 end
+@testset "sizehint!" begin
+ A = zeros(40)
+ resize!(A, 1)
+ sizehint!(A, 1)
+ @test length(A) == 1
+
+ A = zeros(40)
+ resize!(A, 20)
+ sizehint!(A, 1)
+ @test length(A) == 20
+
+ A = [1:10;]
+ sizehint!(A, 20)
+ @test length(A) == length([1:10;])
+ @test A == [1:10;]
+
+ A = [1:10;]
+ resize!(A, 5)
+ sizehint!(A, 5)
+ @test length(A) == 5
+ @test A == [1:5;]
+end
 @testset "reshape with colon" begin
  # Reshape with an omitted dimension
  let A = range(1, stop=60, length=60)