[ConstantFolding] Fold constant calls to log2()

[evandro]

Summary

Somehow, folding calls to log2() with a constant was missing.

[xbolva00]

Ok

[efriedma]

I'm not really happy with adding more uses of ConstantFoldFP/ConstantFoldBinaryFP; they're flawed because they produce results that depend on the host's libm implementation. But I guess there isn't any reason to support log and not log2.

[evandro]

@efriedma, yes, I noticed that and noted it in rL371236.

[efriedma]

I think a few of the functions we use ConstantFoldFP with actually already have implementations in APFloat: floor, ceil, round, fabs, fmod.

I'm not sure "C99" is really relevant; I don't think we support any hosts which don't have C99 libm, anyway.

[evandro]

@efriedma, I never know what Windows supports or not...

llvm/trunk/lib/Analysis/ConstantFolding.cpp

case 'e':
     return Name == "exp" || Name == "expf" ||
            Name == "exp2" || Name == "exp2f";
   case 'f':
     return Name == "fabs" || Name == "fabsf" ||
            Name == "floor" || Name == "floorf" ||
            Name == "fmod" || Name == "fmodf";
   case 'l':
     return Name == "log" || Name == "logf" ||
+           Name == "log2" || Name == "log2f" ||
            Name == "log10" || Name == "log10f";
   case 'n':
     return Name == "nearbyint" || Name == "nearbyintf";
   case 'p':
     return Name == "pow" || Name == "powf";
   case 'r':
     return Name == "rint" || Name == "rintf" ||
            Name == "round" || Name == "roundf";
case LibFunc_floorf:
       break;
     case LibFunc_log:
     case LibFunc_logf:
     case LibFunc_log_finite:
     case LibFunc_logf_finite:
       if (V > 0.0 && TLI->has(Func))
         return ConstantFoldFP(log, V, Ty);
       break;
+    case LibFunc_log2:
+    case LibFunc_log2f:
+    case LibFunc_log2_finite:
+    case LibFunc_log2f_finite:
+      if (V > 0.0 && TLI->has(Func))
+        // TODO: What about hosts that lack a C99 library?
+        return ConstantFoldFP(Log2, V, Ty);
+      break;
     case LibFunc_log10:
     case LibFunc_log10f:
     case LibFunc_log10_finite:
     case LibFunc_log10f_finite:
       if (V > 0.0 && TLI->has(Func))
         // TODO: What about hosts that lack a C99 library?
         return ConstantFoldFP(log10, V, Ty);
       break;

llvm/trunk/test/Analysis/ConstantFolding/math-1.ll

 define double @i_exp2() {
 ; CHECK-LABEL: @i_exp2(
 ; CHECK-NEXT:    ret double 2.000000e+00
 ;
   %res = tail call fast double @llvm.exp2.f64(double 1.0)
   ret double %res
 }
 
-; FIXME
+; FIXME: exp10() is not widely supported.
 declare float @exp10f(float)
 define float @f_exp10f() {
 ; CHECK-LABEL: @f_exp10f(
-; CHECK-NEXT:    [[RES:%.*]] = tail call fast float @exp10f(float 1.000000e+00)
+; CHECK-NEXT:    [[RES:%.*]] = tail call float @exp10f(float 1.000000e+00)
 ; CHECK-NEXT:    ret float [[RES]]
 ;
-  %res = tail call fast float @exp10f(float 1.0)
+  %res = tail call float @exp10f(float 1.0)
   ret float %res
 }
 
 declare double @log(double)
 define double @f_log() {
 ; CHECK-LABEL: @f_log(
 ; CHECK-NEXT:    ret double 0.000000e+00
 ;
   %res = tail call fast double @log(double 1.0)
   ret double %res
 }
 
 declare double @llvm.log.f64(double)
 define double @i_log() {
 ; CHECK-LABEL: @i_log(
 ; CHECK-NEXT:    ret double 0.000000e+00
 ;
   %res = tail call fast double @llvm.log.f64(double 1.0)
   ret double %res
 }
 
-; FIXME
 declare float @log2f(float)
 define float @f_log2f() {
 ; CHECK-LABEL: @f_log2f(
-; CHECK-NEXT:    [[RES:%.*]] = tail call fast float @log2f(float 1.000000e+00)
-; CHECK-NEXT:    ret float [[RES]]
+; CHECK-NEXT:    ret float 0.000000e+00
 ;
   %res = tail call fast float @log2f(float 1.0)
   ret float %res
 }
 
 declare float @llvm.log2.f32(float)
 define float @i_log2f() {
 ; CHECK-LABEL: @i_log2f(