Suspected bug in escape analysis causing unnecessary heap allocs

[digitalentity]

The demo code: https://github.com/digitalentity/tinygo_gotchas/tree/master/src/alloc_woes

There is a heap alloc in main.go at line 12:

func PerformMath() vec3.T {
	a := vec3.T{1, 2, 3}
	b := vec3.T{4, 5, 6} // <<< TinyGo allocates this on heap

	c := b.Scale(0.5)
	return vec3.Cross(&a, c)
}

The vec3.Cross creates and returns a new object, making a, b and c visibility scope only limited to the PerformMath():

func Cross(a, b *T) T {
	return T{
		a[1]*b[2] - a[2]*b[1],
		a[2]*b[0] - a[0]*b[2],
		a[0]*b[1] - a[1]*b[0],
	}
}

Building with -print-allocs=. confirms the unnecessary heap allocation:

$ make build-alloc-woes 
tinygo version 0.41.1 linux/amd64 (using go version go1.25.0 and LLVM version 20.1.1)
toolchain/tinygo/bin/tinygo build -size=short -opt=1 -panic=trap -gc=conservative -scheduler=tasks -print-allocs=. -target=nucleo-f722ze -o build/demo-.hex ./src/alloc_woes
mode: set
/home/ksharlaimov/dev/helvionics/tinygo_gotchas/src/alloc_woes/main.go:12.1,12.54 1 0
/home/ksharlaimov/.cache/tinygo/goroot-4f69c74a8da2c4abfdf60ed68a17fdb99d38c77006ea7da25c73db3862d5a0c8/src/runtime/baremetal.go:44.1,44.24 1 0
/home/ksharlaimov/.cache/tinygo/goroot-4f69c74a8da2c4abfdf60ed68a17fdb99d38c77006ea7da25c73db3862d5a0c8/src/internal/task/task_stack.go:43.1,43.39 1 0
/home/ksharlaimov/.cache/tinygo/goroot-4f69c74a8da2c4abfdf60ed68a17fdb99d38c77006ea7da25c73db3862d5a0c8/src/internal/task/task_stack.go:75.1,75.13 1 0
   code    data     bss |   flash     ram
  13560    1684    4324 |   15244    6008

A test of PerformMath() function with testing.AllocsPerRun says no allocations:

func TestPerformMathAllocations(t *testing.T) {
	allocs := testing.AllocsPerRun(100, func() {
		PerformMath()
	})
	if allocs > 0 {
		t.Errorf("PerformMath allocated %f times, expected 0", allocs)
	}
}

The concern here is

1. Unnecessary heap allocation and GC pressure, making it hard to write predictable latency and memory usage code.
2. Inconsistent escape analysis behavior between TinyGo compiler and testing environment.

[aykevl]

How did you run the test, with go test or tinygo test?

Note that -print-allocs can sometimes report allocations where there are none (since LLVM manages to optimize them away at a later time). This is a bug that we should fix eventually.