cmd/compile: unnecessary bounds check when slicing after using `slices.Grow` · golang/go#73145

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BugReportNeedsInvestigationPerformancecompiler/runtimehelp wanted

Description

go version go1.24.1 linux/amd64

package pkg

import "slices"

func Foo() {
	var xs []int
	xs = slices.Grow(xs, 4)[:len(xs)+4]
}

Slicing after growing inserts a check for the capacity and a conditional call to runtime.panicSliceAcap

The compiler should be able to tell that we just guaranteed that the slice's capacity is at least len(xs)+4 and shouldn't insert another check.

Tech stack: go
Domain: backendperformance
Issue type: performance
Difficulty: 4
Estimated time: half day
Activity status: fresh
Clarity: clear
Prerequisites: Go compiler internalsSSA prove passbounds check elimination
Newbie friendliness: 30
Research direction: Examine the prove pass in cmd/compile/internal/ssa/prove.go to understand how capacity checks are eliminated. Look for existing rules that prove slice capacity after built in functions like append. Implement a new rule that recognizes the pattern 'slices.Grow(s, n)[:len(s)+n]' and ensures the capacity is at least len(s)+n. Check existing linked issues or CLs for similar optimizations.