py/mpprint: Use a padding buffer on the stack.

AJMansfield · AJMansfield · commit 674d78fcbebe · 2025-09-26T12:50:02.000-04:00
This reworks `mp_print_strn` to use a stack-allocated padding buffer rather than special-cased hardcoded ROM strings in order to reduce code size and improve string formatting performance. Note that this is actually just as performant, even for zeroes and spaces! On my RP2350 Cortex M33 hardware, spaces are about 1% faster for short-padding cases, and 3.4% faster for long-padding cases. I've done some cursory tests for alternate values of `PAD_BUF_SIZE`, but the results definitely won't generalize to other architectures, and probably not even to other implementations of the same architecture. The buffer size of 20 is chosen as the smallest size that easily admits a later implementation of #18092 to support padding with grouping characters, to avoid pessimizing the short-padding cases any more than required. I've also explored alternatives involving using `alloca` for the padding buffer, but the conditionals and fallback logic needed to bound stack usage for the pathological cases end up pessimizing code size beyond what's reasonable for the very marginal additional speed gains. Signed-off-by: Anson Mansfield <amansfield@mantaro.com>
diff --git a/py/mpprint.c b/py/mpprint.c
@@ -40,8 +40,6 @@
 #include "py/formatfloat.h"
 #endif
 
-static const char pad_spaces[] = "                ";
-static const char pad_zeroes[] = "0000000000000000";
 
 static void plat_print_strn(void *env, const char *str, size_t len) {
     (void)env;
@@ -58,62 +56,56 @@ int mp_print_str(const mp_print_t *print, const char *str) {
     return len;
 }
 
+// Efficiently print `count` chars looping over `str[len]`.
+static void print_strn_cycle(const mp_print_t *print, const char *str, size_t len, size_t count) {
+    while (count > len) {
+        print->print_strn(print->data, str, len);
+        count -= len;
+    }
+    if (count > 0) {
+        print->print_strn(print->data, str, count);
+    }
+}
+
+// lcm(4,5) to match stride for both comma and underscore grouping
+#define PAD_BUF_SIZE 20
+
 int mp_print_strn(const mp_print_t *print, const char *str, size_t len, unsigned int flags, char fill, int width) {
     int left_pad = 0;
     int right_pad = 0;
     int pad = width - len;
-    int pad_size;
     int total_chars_printed = 0;
-    const char *pad_chars;
-
-    if (!fill || fill == ' ') {
-        pad_chars = pad_spaces;
-        pad_size = sizeof(pad_spaces) - 1;
-    } else if (fill == '0') {
-        pad_chars = pad_zeroes;
-        pad_size = sizeof(pad_zeroes) - 1;
-    } else {
-        // Other pad characters are fairly unusual, so we'll take the hit
-        // and output them 1 at a time.
-        pad_chars = &fill;
-        pad_size = 1;
-    }
 
-    if (flags & PF_FLAG_CENTER_ADJUST) {
-        left_pad = pad / 2;
-        right_pad = pad - left_pad;
-    } else if (flags & PF_FLAG_LEFT_ADJUST) {
-        right_pad = pad;
-    } else {
-        left_pad = pad;
-    }
+    // alignment ensures fast memset
+    char pad_buf[PAD_BUF_SIZE] __attribute__ ((aligned(4)));
+
+    if (pad > 0) {
+        fill = fill ? fill : ' ';
+        memset(pad_buf, fill, sizeof(pad_buf));
+
+        if (flags & PF_FLAG_CENTER_ADJUST) {
+            left_pad = pad / 2;
+            right_pad = pad - left_pad;
+        } else if (flags & PF_FLAG_LEFT_ADJUST) {
+            left_pad = 0;
+            right_pad = pad;
+        } else {
+            left_pad = pad;
+            right_pad = 0;
+        }
 
-    if (left_pad > 0) {
+        print_strn_cycle(print, pad_buf, sizeof(pad_buf), left_pad);
         total_chars_printed += left_pad;
-        while (left_pad > 0) {
-            int p = left_pad;
-            if (p > pad_size) {
-                p = pad_size;
-            }
-            print->print_strn(print->data, pad_chars, p);
-            left_pad -= p;
-        }
     }
-    if (len) {
+
+    if (MP_LIKELY(len)) {
         print->print_strn(print->data, str, len);
         total_chars_printed += len;
     }
-    if (right_pad > 0) {
-        total_chars_printed += right_pad;
-        while (right_pad > 0) {
-            int p = right_pad;
-            if (p > pad_size) {
-                p = pad_size;
-            }
-            print->print_strn(print->data, pad_chars, p);
-            right_pad -= p;
-        }
-    }
+
+    print_strn_cycle(print, pad_buf, sizeof(pad_buf), right_pad);
+    total_chars_printed += right_pad;
+
     return total_chars_printed;
 }
 
