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Fix slab prefetch address on Pop fast path. #715

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Merged
copybara-service merged 1 commit into from
Sep 11, 2026
+10 −10
Merged

Fix slab prefetch address on Pop fast path. #715

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16 changes: 8 additions & 8 deletions tcmalloc/internal/percpu_tcmalloc.h
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Original file line number Diff line number Diff line change
Expand Up @@ -835,7 +835,7 @@ inline ABSL_ATTRIBUTE_ALWAYS_INLINE void* TcmallocSlab<NumClasses>::Pop(
TC_ASSERT_NE(size_class, 0);
void* next;
void* result;
uintptr_t scratch, current;
uintptr_t tcmalloc_slabs_addr, current;

asm goto(TCMALLOC_RSEQ_PROLOGUE(TcmallocSlab_Internal_Pop)
// scratch = tcmalloc_slabs;
Expand All @@ -855,7 +855,7 @@ inline ABSL_ATTRIBUTE_ALWAYS_INLINE void* TcmallocSlab<NumClasses>::Pop(
"subl $0xffff0001, (%[scratch], %[size_class], 4)\n"
// Commit
"5:\n"
: [result] "=&r"(result), [scratch] "=&r"(scratch),
: [result] "=&r"(result), [scratch] "=&r"(tcmalloc_slabs_addr),
[current] "=&r"(current), [next] "=&r"(next)
: TCMALLOC_RSEQ_INPUTS, [begin_mark_mask] "n"(kBeginMark),
[size_class] "r"(size_class)
Expand All @@ -865,9 +865,9 @@ inline ABSL_ATTRIBUTE_ALWAYS_INLINE void* TcmallocSlab<NumClasses>::Pop(
TC_ASSERT(result);
TSANAcquire(result);

// The next pop will be from current-1, but because we prefetch the previous
// element we've already just read that, so prefetch current-2.
PrefetchSlabMemory(scratch + (current - 2) * sizeof(void*));
// The next pop will be from current-2, but because we prefetch the previous
// element we've already just read that, so prefetch current-3.
PrefetchSlabMemory(tcmalloc_slabs_addr + (current - 3) * sizeof(void*));
PrefetchNextObject(next);
return AssumeNotNull(result);
underflow_path:
Expand All @@ -884,7 +884,7 @@ inline ABSL_ATTRIBUTE_ALWAYS_INLINE void* TcmallocSlab<NumClasses>::Pop(
void* region_start;
void* prefetch;
uintptr_t scratch;
uintptr_t current;
uintptr_t current_plus_slabs_addr;
asm goto(
TCMALLOC_RSEQ_PROLOGUE(TcmallocSlab_Internal_Pop)
// region_start = tcmalloc_slabs;
Expand All @@ -908,7 +908,7 @@ inline ABSL_ATTRIBUTE_ALWAYS_INLINE void* TcmallocSlab<NumClasses>::Pop(
// Commit
"5:\n"
: [result] "=&r"(result), [prefetch] "=&r"(prefetch),
[current] "=&r"(current),
[current] "=&r"(current_plus_slabs_addr),
// Temps
[region_start] "=&r"(region_start), [scratch] "=&r"(scratch)
// Real inputs
Expand All @@ -921,7 +921,7 @@ inline ABSL_ATTRIBUTE_ALWAYS_INLINE void* TcmallocSlab<NumClasses>::Pop(

// The next pop will be from current-2, but because we prefetch the previous
// element we've already just read that, so prefetch current-3.
PrefetchSlabMemory(current - 3 * sizeof(void*));
PrefetchSlabMemory(current_plus_slabs_addr - 3 * sizeof(void*));
PrefetchNextObject(prefetch);
return AssumeNotNull(result);
underflow_path:
Expand Down
2 changes: 1 addition & 1 deletion tcmalloc/testing/fast_path.release+insecure.golden
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Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@ new(align)->size - - 5 - 6 - 7 - - - 2 -
new(align,cold)->size - 1 6 - 6 - 8 - - - 2 - 1
new(align,nothrow) - - 5 - 6 - 6 - - - 2 - 1
new(cold)->size - 1 4 - 4 - 7 - - - 2 - 1
new(cold,token_1)->size - 1 4 - 4 - 7 - - - 2 - 1
new(cold,token_1)->size - - 4 - 5 - 7 - - - 2 - 1
new(nothrow) - - 3 - 4 - 5 - - - 2 - 1
new(token_1) - - 3 - 4 - 5 - - - 2 - 1
new(token_1)->size - - 3 - 4 - 6 - - - 2 - 1
Expand Down
2 changes: 1 addition & 1 deletion tcmalloc/testing/fast_path.release.golden
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Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@ new(align)->size - - 5 - 6 - 7 - - - 2 -
new(align,cold)->size - 1 6 - 6 - 8 - - - 2 - 1
new(align,nothrow) - - 5 - 6 - 6 - - - 2 - 1
new(cold)->size - 1 4 - 4 - 7 - - - 2 - 1
new(cold,token_1)->size - 1 4 - 4 - 7 - - - 2 - 1
new(cold,token_1)->size - - 4 - 5 - 7 - - - 2 - 1
new(nothrow) - - 3 - 4 - 5 - - - 2 - 1
new(token_1) - - 3 - 4 - 5 - - - 2 - 1
new(token_1)->size - - 3 - 4 - 6 - - - 2 - 1
Expand Down
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