diff --git a/http.go b/http.go
index 5d8dc93477..595eafa9c2 100644
--- a/http.go
+++ b/http.go
@@ -8,7 +8,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"math"
 	"mime/multipart"
 	"net"
 	"os"
@@ -2210,7 +2209,7 @@ func readBodyIdentity(r *bufio.Reader, maxBodySize int, dst []byte) ([]byte, err
 			return dst[:offset], ErrBodyTooLarge
 		}
 		if len(dst) == offset {
-			n := round2(2 * offset)
+			n := roundUpForSliceCap(2 * offset)
 			if maxBodySize > 0 && n > maxBodySize {
 				n = maxBodySize + 1
 			}
@@ -2229,7 +2228,7 @@ func appendBodyFixedSize(r *bufio.Reader, dst []byte, n int) ([]byte, error) {
 	offset := len(dst)
 	dstLen := offset + n
 	if cap(dst) < dstLen {
-		b := make([]byte, round2(dstLen))
+		b := make([]byte, roundUpForSliceCap(dstLen))
 		copy(b, dst)
 		dst = b
 	}
@@ -2339,26 +2338,6 @@ func readCrLf(r *bufio.Reader) error {
 	return nil
 }
 
-func round2(n int) int {
-	if n <= 0 {
-		return 0
-	}
-
-	x := uint32(n - 1)
-	x |= x >> 1
-	x |= x >> 2
-	x |= x >> 4
-	x |= x >> 8
-	x |= x >> 16
-
-	// Make sure we don't return 0 due to overflow, even on 32 bit systems
-	if x >= uint32(math.MaxInt32) {
-		return math.MaxInt32
-	}
-
-	return int(x + 1)
-}
-
 // SetTimeout sets timeout for the request.
 //
 // req.SetTimeout(t); c.Do(&req, &resp) is equivalent to
diff --git a/http_test.go b/http_test.go
index d4717f640c..370e9aa9e5 100644
--- a/http_test.go
+++ b/http_test.go
@@ -16,6 +16,7 @@ import (
 	"strings"
 	"testing"
 	"time"
+	"unsafe"
 
 	"github.com/valyala/bytebufferpool"
 )
@@ -1967,25 +1968,31 @@ func testSetResponseBodyStreamChunked(t *testing.T, body string, trailer map[str
 	}
 }
 
-func TestRound2(t *testing.T) {
+func TestRound2ForSliceCap(t *testing.T) {
 	t.Parallel()
 
-	testRound2(t, 0, 0)
-	testRound2(t, 1, 1)
-	testRound2(t, 2, 2)
-	testRound2(t, 3, 4)
-	testRound2(t, 4, 4)
-	testRound2(t, 5, 8)
-	testRound2(t, 7, 8)
-	testRound2(t, 8, 8)
-	testRound2(t, 9, 16)
-	testRound2(t, 0x10001, 0x20000)
-	testRound2(t, math.MaxInt32-1, math.MaxInt32)
+	testRound2ForSliceCap(t, 0, 0)
+	testRound2ForSliceCap(t, 1, 1)
+	testRound2ForSliceCap(t, 2, 2)
+	testRound2ForSliceCap(t, 3, 4)
+	testRound2ForSliceCap(t, 4, 4)
+	testRound2ForSliceCap(t, 5, 8)
+	testRound2ForSliceCap(t, 7, 8)
+	testRound2ForSliceCap(t, 8, 8)
+	testRound2ForSliceCap(t, 9, 16)
+	testRound2ForSliceCap(t, 0x10001, 0x20000)
+
+	if unsafe.Sizeof(int(0)) == 4 {
+		testRound2ForSliceCap(t, math.MaxInt32-1, math.MaxInt32)
+	} else {
+		testRound2ForSliceCap(t, math.MaxInt32, math.MaxInt32)
+		testRound2ForSliceCap(t, math.MaxInt64-1, math.MaxInt64-1)
+	}
 }
 
-func testRound2(t *testing.T, n, expectedRound2 int) {
-	if round2(n) != expectedRound2 {
-		t.Fatalf("Unexpected round2(%d)=%d. Expected %d", n, round2(n), expectedRound2)
+func testRound2ForSliceCap(t *testing.T, n, expectedRound2 int) {
+	if roundUpForSliceCap(n) != expectedRound2 {
+		t.Fatalf("Unexpected round2(%d)=%d. Expected %d", n, roundUpForSliceCap(n), expectedRound2)
 	}
 }
 
diff --git a/round2_32.go b/round2_32.go
new file mode 100644
index 0000000000..541b85e216
--- /dev/null
+++ b/round2_32.go
@@ -0,0 +1,29 @@
+//go:build !amd64 && !arm64 && !ppc64 && !ppc64le && !s390x
+// +build !amd64,!arm64,!ppc64,!ppc64le,!s390x
+
+package fasthttp
+
+func roundUpForSliceCap(n int) int {
+	if n <= 0 {
+		return 0
+	}
+
+	// Above 100MB, we don't round up as the overhead is too large.
+	if n > 100*1024*1024 {
+		return n
+	}
+
+	x := uint32(n - 1)
+	x |= x >> 1
+	x |= x >> 2
+	x |= x >> 4
+	x |= x >> 8
+	x |= x >> 16
+
+	// Make sure we don't return 0 due to overflow, even on 32 bit systems
+	if x >= uint32(math.MaxInt32) {
+		return math.MaxInt32
+	}
+
+	return int(x + 1)
+}
diff --git a/round2_64.go b/round2_64.go
new file mode 100644
index 0000000000..8a8e2a23e1
--- /dev/null
+++ b/round2_64.go
@@ -0,0 +1,24 @@
+//go:build amd64 || arm64 || ppc64 || ppc64le || s390x
+// +build amd64 arm64 ppc64 ppc64le s390x
+
+package fasthttp
+
+func roundUpForSliceCap(n int) int {
+	if n <= 0 {
+		return 0
+	}
+
+	// Above 100MB, we don't round up as the overhead is too large.
+	if n > 100*1024*1024 {
+		return n
+	}
+
+	x := uint64(n - 1)
+	x |= x >> 1
+	x |= x >> 2
+	x |= x >> 4
+	x |= x >> 8
+	x |= x >> 16
+
+	return int(x + 1)
+}