Add snippet

README.md

@@ -1,18 +1,21 @@
 # c_bitfields_integer_promotion
-Little experiment to check how integer promotion affects bitfields
 
-The interest in this came after the discussion at https://lore.kernel.org/all/[email protected]/t/#u
+Little experiment to check how integer promotion affects bitfields.
+
+The interest in this came after the discussion at <https://lore.kernel.org/all/[email protected]/t/#u>
+
+Turns out that for bitfields bigger than `sizeof(int)` the behavior is compiler dependent.
 
 ## A bunch of related links
 
-https://gcc.gnu.org/onlinedocs/gcc/Structures-unions-enumerations-and-bit-fields-implementation.html
+<https://gcc.gnu.org/onlinedocs/gcc/Structures-unions-enumerations-and-bit-fields-implementation.html>
 
-https://stackoverflow.com/questions/2647320/struct-bitfield-max-size-c99-c
+<https://stackoverflow.com/questions/2647320/struct-bitfield-max-size-c99-c>
 
-https://www.gnu.org/software/gnu-c-manual/gnu-c-manual.html#Bit-Fields
+<https://www.gnu.org/software/gnu-c-manual/gnu-c-manual.html#Bit-Fields>
 
-https://archive.org/details/the-ansi-c-programming-language-by-brian-w.-kernighan-dennis-m.-ritchie.org/page/132/mode/2up
+<https://archive.org/details/the-ansi-c-programming-language-by-brian-w.-kernighan-dennis-m.-ritchie.org/page/132/mode/2up>
 
-https://stackoverflow.com/questions/32529080/should-bit-fields-less-than-int-in-size-be-the-subject-of-integral-promotion
+<https://stackoverflow.com/questions/32529080/should-bit-fields-less-than-int-in-size-be-the-subject-of-integral-promotion>
 
-https://stackoverflow.com/questions/46073295/implicit-type-promotion-rules
+<https://stackoverflow.com/questions/46073295/implicit-type-promotion-rules>

main.c

@@ -0,0 +1,33 @@
+#include <stdio.h>
+
+struct s1{
+  unsigned int a: 20;
+  unsigned int _: 12;
+};
+
+struct s2 {
+  unsigned long long b: 40;
+  unsigned long long _: 24;
+};
+
+int main()
+{
+    struct s1 s1;
+    struct s2 s2;
+
+    s1.a = 2;
+    s2.b = 2;
+
+    unsigned int a = s1.a << 19;
+    unsigned long long b = s2.b << 39;
+
+    printf("0x%x\n", a);    //both gcc and clang: 0x100000
+    printf("0x%llx\n", b);  //gcc: 0x0  clang: 0x10000000000
+
+    return 0;
+}
+
+/*
+Debian clang version 18.1.8
+gcc (Debian 12.2.0-14) 12.2.0
+*/