[dev.ssa] cmd/compile/internal/ssa: transform degenerate control blocks
* In cases where we end up with empty branches like in
if a then jmp b else jmp b;
the flow can be replaced by a; jmp b.
The following functions is optimized as follows:
func f(a bool, x int) int {
v := 0
if a {
v = -1
} else {
v = -1
}
return x | v
}
Before this change:
02819 (arith_ssa.go:362) VARDEF "".~r2+16(FP)
02820 (arith_ssa.go:362) MOVQ $0, "".~r2+16(FP)
02821 (arith_ssa.go:362) MOVB "".a(FP), AX
02822 (arith_ssa.go:362) TESTB AX, AX
02823 (arith_ssa.go:364) JEQ 2824
02824 (arith_ssa.go:369) VARDEF "".~r2+16(FP)
02825 (arith_ssa.go:369) MOVQ $-1, "".~r2+16(FP)
02826 (arith_ssa.go:369) RET
After this change:
02819 (arith_ssa.go:362) VARDEF "".~r2+16(FP)
02820 (arith_ssa.go:369) VARDEF "".~r2+16(FP)
02821 (arith_ssa.go:369) MOVQ $-1, "".~r2+16(FP)
02822 (arith_ssa.go:369) RET
Updates #14277
Change-Id: Ibe7d284f43406c704903632a4fcf2a4a64059686
Reviewed-on: https://go-review.googlesource.com/19464
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>