diff --git a/xdiff/xdiffi.c b/xdiff/xdiffi.c
index 53e803e6bc..8c64519eac 100644
--- a/xdiff/xdiffi.c
+++ b/xdiff/xdiffi.c
@@ -326,7 +326,7 @@ int xdl_do_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
 	}
 
 	if (XDF_DIFF_ALG(xpp->flags) == XDF_HISTOGRAM_DIFF) {
-		res = xdl_do_histogram_diff(mf1, mf2, xpp, xe);
+		res = xdl_do_histogram_diff(xpp, xe);
 		goto out;
 	}
 
diff --git a/xdiff/xdiffi.h b/xdiff/xdiffi.h
index 8f1c7c8b04..9d988e0263 100644
--- a/xdiff/xdiffi.h
+++ b/xdiff/xdiffi.h
@@ -58,7 +58,6 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
 		  xdemitconf_t const *xecfg);
 int xdl_do_patience_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
 		xdfenv_t *env);
-int xdl_do_histogram_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
-		xdfenv_t *env);
+int xdl_do_histogram_diff(xpparam_t const *xpp, xdfenv_t *env);
 
 #endif /* #if !defined(XDIFFI_H) */
diff --git a/xdiff/xhistogram.c b/xdiff/xhistogram.c
index df909004c1..16a8fe2f3f 100644
--- a/xdiff/xhistogram.c
+++ b/xdiff/xhistogram.c
@@ -362,8 +362,7 @@ static int histogram_diff(xpparam_t const *xpp, xdfenv_t *env,
 	return result;
 }
 
-int xdl_do_histogram_diff(mmfile_t *file1, mmfile_t *file2,
-	xpparam_t const *xpp, xdfenv_t *env)
+int xdl_do_histogram_diff(xpparam_t const *xpp, xdfenv_t *env)
 {
 	return histogram_diff(xpp, env,
 		env->xdf1.dstart + 1, env->xdf1.dend - env->xdf1.dstart + 1,