Alive2 reports a few incorrect optimizations around select w/ fast-math in Transforms/InstCombine/minmax-fp.ll: ----------------------------------------- nsz allows the output to be +0/-0 non-deterministically. In this example, the source always returns +0 and the target may return +0 or -0. The select in src would need nsz as well: define <2 x float> @fsub_fmax(<2 x float> %x, <2 x float> %y) { %n1 = fsub <2 x float> { -0.000000, -0.000000 }, %x %n2 = fsub <2 x float> { -0.000000, -0.000000 }, %y %cond = fcmp nnan nsz uge <2 x float> %n1, %n2 %max = select <2 x i1> %cond, <2 x float> %n1, <2 x float> %n2 ret <2 x float> %max } => define <2 x float> @fsub_fmax(<2 x float> %x, <2 x float> %y) { %cond.inv = fcmp nnan nsz ogt <2 x float> %x, %y %1 = select nnan nsz <2 x i1> %cond.inv, <2 x float> %y, <2 x float> %x %max = fneg <2 x float> %1 ret <2 x float> %max } Transformation doesn't verify! ERROR: Value mismatch Example: <2 x float> %x = < poison, #x80000000 (-0.0) > <2 x float> %y = < poison, #x00800001 (0.000000000000?) > Source: <2 x float> %n1 = < poison, #x00000000 (+0.0) > <2 x float> %n2 = < poison, #x80800001 (-0.000000000000?) > <2 x i1> %cond = < poison, #x1 (1) > <2 x float> %max = < poison, #x00000000 (+0.0) > Target: <2 x i1> %cond.inv = < poison, #x0 (0) > <2 x float> %1 = < poison, #x00000000 (+0.0) > <2 x float> %max = < poison, #x80000000 (-0.0) > Source value: < poison, #x00000000 (+0.0) > Target value: < poison, #x80000000 (-0.0) > ----------------------------- select nnan only applies to the chosen operand (?). If that's the case then the optimization below isn't correct for a NaN input. If nnan applies to both select operands, then fptrunc_select_true_val_extra_use in fptrunc.ll is incorrect. define float @maxnum_ogt_fmf_on_select(float %a, float %b) { %cond = fcmp ogt float %a, %b %f = select nnan nsz i1 %cond, float %a, float %b ret float %f } => define float @maxnum_ogt_fmf_on_select(float %a, float %b) { %1 = fmax nnan nsz float %a, %b ret float %1 } Transformation doesn't verify! ERROR: Target is more poisonous than source Example: float %a = NaN float %b = #x20008b45 (0.000000000000?) Source: i1 %cond = #x0 (0) float %f = #x20008b45 (0.000000000000?) Target: float %1 = poison Source value: #x20008b45 (0.000000000000?) Target value: poison