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Author	SHA1	Message	Date
thepudds	4879151d1d	cmd/compile: introduce alias analysis and automatically free non-aliased memory after growslice ... This CL is part of a set of CLs that attempt to reduce how much work the GC must do. See the design in https://go.dev/design/74299-runtime-freegc This CL updates the compiler to examine append calls to prove whether or not the slice is aliased. If proven unaliased, the compiler automatically inserts a call to a new runtime function introduced with this CL, runtime.growsliceNoAlias, which frees the old backing memory immediately after slice growth is complete and the old storage is logically dead. Two append benchmarks below show promising results, executing up to ~2x faster and up to factor of ~3 memory reduction with this CL. The approach works with multiple append calls for the same slice, including inside loops, and the final slice memory can be escaping, such as in a classic pattern of returning a slice from a function after the slice is built. (The final slice memory is never freed with this CL, though we have other work that tackles that.) An example target for this CL is we automatically free the intermediate memory for the appends in the loop in this function: func f1(input []int) []int { var s []int for _, x := range input { s = append(s, g(x)) // s cannot be aliased here if h(x) { s = append(s, x) // s cannot be aliased here } } return s // slice escapes at end } In this case, the compiler and the runtime collaborate so that the heap allocated backing memory for s is automatically freed after a successful grow. (For the first grow, there is nothing to free, but for the second and subsequent growths, the old heap memory is freed automatically.) The new runtime.growsliceNoAlias is primarily implemented by calling runtime.freegc, which we introduced in CL 673695. The high-level approach here is we step through the IR starting from a slice declaration and look for any operations that either alias the slice or might do so, and treat any IR construct we don't specifically handle as a potential alias (and therefore conservatively fall back to treating the slice as aliased when encountering something not understood). For loops, some additional care is required. We arrange the analysis so that an alias in the body of a loop causes all the appends in that same loop body to be marked aliased, even if the aliasing occurs after the append in the IR: func f2() { var s []int for i := range 10 { s = append(s, i) // aliased due to next line alias = s } } For nested loops, we analyse the nesting appropriately so that for example this append is still proven as non-aliased in the inner loop even though it aliased for the outer loop: func f3() { for range 10 { var s []int for i := range 10 { s = append(s, i) // append using non-aliased slice } alias = s } } A good starting point is the beginning of the test/escape_alias.go file, which starts with ~10 introductory examples with brief comments that attempt to illustrate the high-level approach. For more details, see the new .../internal/escape/alias.go file, especially the (*aliasAnalysis).analyze method. In the first benchmark, an append in a loop builds up a slice from nothing, where the slice elements are each 64 bytes. In the table below, 'count' is the number of appends. With 1 append, there is no opportunity for this CL to free memory. Once there are 2 appends, the growth from 1 element to 2 elements means the compiler-inserted growsliceNoAlias frees the 1-element array, and we see a ~33% reduction in memory use and a small reported speed improvement. As the number of appends increases for example to 5, we are at a ~20% speed improvement and ~45% memory reduction, and so on until we reach ~40% faster and ~50% less memory allocated at the end of the table. There can be variation in the reported numbers based on -randlayout, so this table is for 30 different values of -randlayout with a total n=150. (Even so, there is still some variation, so we probably should not read too much into small changes.) This is with GOAMD64=v3 on a VM that gcc reports is cascadelake. goos: linux goarch: amd64 pkg: runtime cpu: Intel(R) Xeon(R) CPU @ 2.80GHz │ old-1bb1f2bf0c │ freegc-8ba7421-ps16 │ │ sec/op │ sec/op vs base │ Append64Bytes/count=1-4 31.09n ± 2% 31.69n ± 1% +1.95% (n=150) Append64Bytes/count=2-4 73.31n ± 1% 70.27n ± 0% -4.15% (n=150) Append64Bytes/count=3-4 142.7n ± 1% 124.6n ± 1% -12.68% (n=150) Append64Bytes/count=4-4 149.6n ± 1% 127.7n ± 0% -14.64% (n=150) Append64Bytes/count=5-4 277.1n ± 1% 213.6n ± 0% -22.90% (n=150) Append64Bytes/count=6-4 280.7n ± 1% 216.5n ± 1% -22.87% (n=150) Append64Bytes/count=10-4 544.3n ± 1% 386.6n ± 0% -28.97% (n=150) Append64Bytes/count=20-4 1058.5n ± 1% 715.6n ± 1% -32.39% (n=150) Append64Bytes/count=50-4 2.121µ ± 1% 1.404µ ± 1% -33.83% (n=150) Append64Bytes/count=100-4 4.152µ ± 1% 2.736µ ± 1% -34.11% (n=150) Append64Bytes/count=200-4 7.753µ ± 1% 4.882µ ± 1% -37.03% (n=150) Append64Bytes/count=400-4 15.163µ ± 2% 9.273µ ± 1% -38.84% (n=150) geomean 601.8n 455.0n -24.39% │ old-1bb1f2bf0c │ freegc-8ba7421-ps16 │ │ B/op │ B/op vs base │ Append64Bytes/count=1-4 64.00 ± 0% 64.00 ± 0% ~ (n=150) Append64Bytes/count=2-4 192.0 ± 0% 128.0 ± 0% -33.33% (n=150) Append64Bytes/count=3-4 448.0 ± 0% 256.0 ± 0% -42.86% (n=150) Append64Bytes/count=4-4 448.0 ± 0% 256.0 ± 0% -42.86% (n=150) Append64Bytes/count=5-4 960.0 ± 0% 512.0 ± 0% -46.67% (n=150) Append64Bytes/count=6-4 960.0 ± 0% 512.0 ± 0% -46.67% (n=150) Append64Bytes/count=10-4 1.938Ki ± 0% 1.000Ki ± 0% -48.39% (n=150) Append64Bytes/count=20-4 3.938Ki ± 0% 2.001Ki ± 0% -49.18% (n=150) Append64Bytes/count=50-4 7.938Ki ± 0% 4.005Ki ± 0% -49.54% (n=150) Append64Bytes/count=100-4 15.938Ki ± 0% 8.021Ki ± 0% -49.67% (n=150) Append64Bytes/count=200-4 31.94Ki ± 0% 16.08Ki ± 0% -49.64% (n=150) Append64Bytes/count=400-4 63.94Ki ± 0% 32.33Ki ± 0% -49.44% (n=150) geomean 1.991Ki 1.124Ki -43.54% │ old-1bb1f2bf0c │ freegc-8ba7421-ps16 │ │ allocs/op │ allocs/op vs base │ Append64Bytes/count=1-4 1.000 ± 0% 1.000 ± 0% ~ (n=150) Append64Bytes/count=2-4 2.000 ± 0% 1.000 ± 0% -50.00% (n=150) Append64Bytes/count=3-4 3.000 ± 0% 1.000 ± 0% -66.67% (n=150) Append64Bytes/count=4-4 3.000 ± 0% 1.000 ± 0% -66.67% (n=150) Append64Bytes/count=5-4 4.000 ± 0% 1.000 ± 0% -75.00% (n=150) Append64Bytes/count=6-4 4.000 ± 0% 1.000 ± 0% -75.00% (n=150) Append64Bytes/count=10-4 5.000 ± 0% 1.000 ± 0% -80.00% (n=150) Append64Bytes/count=20-4 6.000 ± 0% 1.000 ± 0% -83.33% (n=150) Append64Bytes/count=50-4 7.000 ± 0% 1.000 ± 0% -85.71% (n=150) Append64Bytes/count=100-4 8.000 ± 0% 1.000 ± 0% -87.50% (n=150) Append64Bytes/count=200-4 9.000 ± 0% 1.000 ± 0% -88.89% (n=150) Append64Bytes/count=400-4 10.000 ± 0% 1.000 ± 0% -90.00% (n=150) geomean 4.331 1.000 -76.91% The second benchmark is similar, but instead uses an 8-byte integer for the slice element. The first 4 appends in the loop never call into the runtime thanks to the excellent CL 664299 introduced by Keith in Go 1.25 that allows some <= 32 byte dynamically-sized slices to be on the stack, so this CL is neutral for <= 32 bytes. Once the 5th append occurs at count=5, a grow happens via the runtime and heap allocates as normal, but freegc does not yet have anything to free, so we see a small ~1.4ns penalty reported there. But once the second growth happens, the older heap memory is now automatically freed by freegc, so we start to see some benefit in memory reductions and speed improvements, starting at a tiny speed improvement (close to a wash, or maybe noise) by the second growth before count=10, and building up to ~2x faster with ~68% fewer allocated bytes reported. goos: linux goarch: amd64 pkg: runtime cpu: Intel(R) Xeon(R) CPU @ 2.80GHz │ old-1bb1f2bf0c │ freegc-8ba7421-ps16 │ │ sec/op │ sec/op vs base │ AppendInt/count=1-4 2.978n ± 0% 2.969n ± 0% -0.30% (p=0.000 n=150) AppendInt/count=4-4 4.292n ± 3% 4.163n ± 3% ~ (p=0.528 n=150) AppendInt/count=5-4 33.50n ± 0% 34.93n ± 0% +4.25% (p=0.000 n=150) AppendInt/count=10-4 76.21n ± 1% 75.67n ± 0% -0.72% (p=0.000 n=150) AppendInt/count=20-4 150.6n ± 1% 133.0n ± 0% -11.65% (n=150) AppendInt/count=50-4 284.1n ± 1% 225.6n ± 0% -20.59% (n=150) AppendInt/count=100-4 544.2n ± 1% 392.4n ± 1% -27.89% (n=150) AppendInt/count=200-4 1051.5n ± 1% 702.3n ± 0% -33.21% (n=150) AppendInt/count=400-4 2.041µ ± 1% 1.312µ ± 1% -35.70% (n=150) AppendInt/count=1000-4 5.224µ ± 2% 2.851µ ± 1% -45.43% (n=150) AppendInt/count=2000-4 11.770µ ± 1% 6.010µ ± 1% -48.94% (n=150) AppendInt/count=3000-4 17.747µ ± 2% 8.264µ ± 1% -53.44% (n=150) geomean 331.8n 246.4n -25.72% │ old-1bb1f2bf0c │ freegc-8ba7421-ps16 │ │ B/op │ B/op vs base │ AppendInt/count=1-4 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=150) AppendInt/count=4-4 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=150) AppendInt/count=5-4 64.00 ± 0% 64.00 ± 0% ~ (p=1.000 n=150) AppendInt/count=10-4 192.0 ± 0% 128.0 ± 0% -33.33% (n=150) AppendInt/count=20-4 448.0 ± 0% 256.0 ± 0% -42.86% (n=150) AppendInt/count=50-4 960.0 ± 0% 512.0 ± 0% -46.67% (n=150) AppendInt/count=100-4 1.938Ki ± 0% 1.000Ki ± 0% -48.39% (n=150) AppendInt/count=200-4 3.938Ki ± 0% 2.001Ki ± 0% -49.18% (n=150) AppendInt/count=400-4 7.938Ki ± 0% 4.005Ki ± 0% -49.54% (n=150) AppendInt/count=1000-4 24.56Ki ± 0% 10.05Ki ± 0% -59.07% (n=150) AppendInt/count=2000-4 58.56Ki ± 0% 20.31Ki ± 0% -65.32% (n=150) AppendInt/count=3000-4 85.19Ki ± 0% 27.30Ki ± 0% -67.95% (n=150) geomean ² -42.81% │ old-1bb1f2bf0c │ freegc-8ba7421-ps16 │ │ allocs/op │ allocs/op vs base │ AppendInt/count=1-4 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=150) AppendInt/count=4-4 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=150) AppendInt/count=5-4 1.000 ± 0% 1.000 ± 0% ~ (p=1.000 n=150) AppendInt/count=10-4 2.000 ± 0% 1.000 ± 0% -50.00% (n=150) AppendInt/count=20-4 3.000 ± 0% 1.000 ± 0% -66.67% (n=150) AppendInt/count=50-4 4.000 ± 0% 1.000 ± 0% -75.00% (n=150) AppendInt/count=100-4 5.000 ± 0% 1.000 ± 0% -80.00% (n=150) AppendInt/count=200-4 6.000 ± 0% 1.000 ± 0% -83.33% (n=150) AppendInt/count=400-4 7.000 ± 0% 1.000 ± 0% -85.71% (n=150) AppendInt/count=1000-4 9.000 ± 0% 1.000 ± 0% -88.89% (n=150) AppendInt/count=2000-4 11.000 ± 0% 1.000 ± 0% -90.91% (n=150) AppendInt/count=3000-4 12.000 ± 0% 1.000 ± 0% -91.67% (n=150) geomean ² -72.76% ² Of course, these are just microbenchmarks, but likely indicate there are some opportunities here. The immediately following CL 712422 tackles inlining and is able to get runtime.freegc working automatically with iterators such as used by slices.Collect, which becomes able to automatically free the intermediate memory from its repeated appends (which earlier in this work required a temporary hand edit to the slices package). For now, we only use the NoAlias version for element types without pointers while waiting on additional runtime support in CL 698515. Updates #74299 Change-Id: I1b9d286aa97c170dcc2e203ec0f8ca72d84e8221 Reviewed-on: https://go-review.googlesource.com/c/go/+/710015 Reviewed-by: Keith Randall <khr@google.com> Auto-Submit: Keith Randall <khr@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@golang.org>	2025-11-26 19:04:05 -08:00