[PATCH] Simplify switch table lookup if a linear mapping is possible

[Erik Eckstein]

Hi Hans,

this patch adds a simple optimization for switch table lookup: It computes the output value directly with an (optional) mul and add if there is a linear mapping between index and output. Example:

int f1(int x) {
	switch (x) {
		case 0: return 10;
		case 1: return 11;
		case 2: return 12;
		case 3: return 13;
	}
	return 0;
}

generates:

define i32 @f1(i32 %x) #0 {
entry:
  %0 = icmp ult i32 %x, 4
  br i1 %0, label %switch.lookup, label %return

switch.lookup:                                    ; preds = %entry
  %switch.offset = add i32 %x, 10
  ret i32 %switch.offset

return:                                           ; preds = %entry
  ret i32 0
}

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Can you (or anyone else) please review it.

Thanks,
Erik