gallivm: add a log function that handles edge cases

Same as log2_safe, which means that it can handle infs, 0s and
nans.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/gallium/auxiliary/gallivm/lp_bld_arit.c

@@ -3080,6 +3080,7 @@ lp_build_exp(struct lp_build_context *bld,
 
 /**
  * Generate log(x)
+ * Behavior is undefined with infs, 0s and nans
  */
 LLVMValueRef
 lp_build_log(struct lp_build_context *bld,
@@ -3094,6 +3095,22 @@ lp_build_log(struct lp_build_context *bld,
    return lp_build_mul(bld, log2, lp_build_log2(bld, x));
 }
 
+/**
+ * Generate log(x) that handles edge cases (infs, 0s and nans)
+ */
+LLVMValueRef
+lp_build_log_safe(struct lp_build_context *bld,
+                  LLVMValueRef x)
+{
+   /* log(2) */
+   LLVMValueRef log2 = lp_build_const_vec(bld->gallivm, bld->type,
+                                          0.69314718055994529);
+
+   assert(lp_check_value(bld->type, x));
+
+   return lp_build_mul(bld, log2, lp_build_log2_safe(bld, x));
+}
+
 
 /**
  * Generate polynomial.

src/gallium/auxiliary/gallivm/lp_bld_arit.h

@@ -291,6 +291,10 @@ LLVMValueRef
 lp_build_log(struct lp_build_context *bld,
              LLVMValueRef a);
 
+LLVMValueRef
+lp_build_log_safe(struct lp_build_context *bld,
+                  LLVMValueRef a);
+
 LLVMValueRef
 lp_build_exp2(struct lp_build_context *bld,
               LLVMValueRef a);