For example:
long long test(long long a, long long b) { if (a << b > 0) return b; if (a << b < 0) return a; return a*b; }
Produces:
# %bb.0: # %entry sld. 5, 3, 4 ble 0, .LBB0_2 # %bb.1: # %return mr 3, 4 blr .LBB0_2: # %if.end cmpldi 5, 0 li 5, 1 isel 4, 4, 5, 2 mulld 3, 4, 3 blr
But the compare (cmpldi 5, 0) is redundant and can be removed (CR0 already contains the result of that comparison).
The root cause of this is that LLVM converts signed comparisons into equality comparison based on dominance. Equality comparisons are unsigned by default, so we get either a record-form or cmp (without the l for logical) feeding a cmpl. That is the situation we want to avoid here.
nit: missing . at the end of the sentence.