[llvm] [InstCombine] Lower ilogb to bit operations for normal values (PR #218044)

[via llvm-commits]

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@@ -199,5 +199,253 @@ define i32 @ilogb_poison() {
   ret i32 %r
 }
 
-declare i32 @ilogbf(float)
+
+; The following tests are for the ilogb normal optimization where floating-point
+; numbers which are known to be normal can have the exponent extracted using
+; simple bit manipulations.
+
+; copysign is used to create a fp value which won't cause the ilogb call to
+; be immediately constant-folded.
+
+define i32 @ilogb_known_normal_non_pow2(double %sign) {
+; CHECK-LABEL: define i32 @ilogb_known_normal_non_pow2(double %sign) {
+; CHECK-NEXT:    ret i32 3
+;
+  %x = call double @llvm.copysign.f64(double 12.5, double %sign)
+  %r = call i32 @ilogb(double %x)
+  ret i32 %r
+}
+
+define i32 @ilogbf_known_normal_non_pow2(float %sign) {
+; CHECK-LABEL: define i32 @ilogbf_known_normal_non_pow2(float %sign) {
+; CHECK-NEXT:    ret i32 3
+;
+  %x = call float @llvm.copysign.f32(float 12.5, float %sign)
+  %r = call i32 @ilogbf(float %x)
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tadeuszjt wrote:

I'm not sure how errno would be tested in this case because ilogb appears to only set errno for non-normal cases.

https://github.com/llvm/llvm-project/pull/218044