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Negate find_object GetConservativeGC logic. #39905

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Merged
jkotas merged 3 commits into from
Jul 28, 2020
Merged

Negate find_object GetConservativeGC logic. #39905

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0817d28
Negate find_object GetConservativeGC logic.
yowl Jul 24, 2020
9e4ab77
conservative gc - tolerate if interior > heap_segment_allocated
yowl Jul 28, 2020
027a8c2
remove duplicate
yowl Jul 28, 2020
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8 changes: 4 additions & 4 deletions src/coreclr/src/gc/gc.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -18088,12 +18088,12 @@ uint8_t* gc_heap::find_object (uint8_t* interior)
{
// this is a pointer to a UOH object
heap_segment* seg = find_segment (interior, FALSE);
if (seg
if (seg)
{
#ifdef FEATURE_CONSERVATIVE_GC
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@jkotas jkotas Jul 24, 2020

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Note that this effects the path we take here in the default CoreCLR config. CoreCLR has FEATURE_CONSERVATIVE_GC defined and GCConfig::GetConservativeGC defaults to false.

It means that this was effectively if (seg && interior <= heap_segment_allocated(seg)) before this change in the default CoreCLR config, and it is just if (seg) after this change.

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@Maoni0 Maoni0 Jul 27, 2020

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these checks for GetConservativeGC seem suspicious to me - I dunno who added them but the impl seems questionable... the idea, if I had to guess, was probably that if we are in conservative mode (ie the config is set), we can tolerate an interior pointer that's > heap_segment_allocated but all we do is we return 0 there - the only case where we could return a non zero value is if an interior pointer is >= heap_segment_mem and < heap_segment_allocated, regardless of whether the conservative config is set. so the code should just be

        if (seg && (interior < heap_segment_allocated(seg))
        {

and the assert can be deleted.

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@Maoni0 Maoni0 Jul 27, 2020

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if we want to be more complete, we could do

#ifdef FEATURE_CONSERVATIVE_GC
            if (interior >= heap_segment_allocated (seg))
                return 0;
#else
            assert (interior < heap_segment_allocated (seg));
#endif

meaning that if we find a valid seg for it, we tolerate it if conservative config is set and inteiror is > heap_segment_allocated.

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@yowl yowl Jul 28, 2020

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Thanks for the comments. I believe I have updated as per the last comment. I tried the Wasm Generics test in a loop 100 times and it completed without errors, so this fix looks good for CoreRT/Wasm. I'm sure you experts will advise if I have it wrong.

&& (GCConfig::GetConservativeGC() || interior <= heap_segment_allocated(seg))
if (interior >= heap_segment_allocated(seg))
return 0;
#endif
)
{
// If interior falls within the first free object at the beginning of a generation,
// we don't have brick entry for it, and we may incorrectly treat it as on large object heap.
int align_const = get_alignment_constant (heap_segment_read_only_p (seg)
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