x86 asm: move gnu gas char literals from x86-assembly-cheat

Author: cirosantilli

README.adoc

@@ -12363,6 +12363,20 @@ When reading disassembly, many instructions have either a `.n` or `.w` suffix.
 
 Bibliography: https://stackoverflow.com/questions/27147043/n-suffix-to-branch-instruction
 
+==== GNU GAS assembler char literals
+
+link:userland/arch/x86_64/char_literals.S[]
+
+http://stackoverflow.com/questions/33246811/how-to-use-character-literals-in-gnu-gas-to-replace-numbers
+
+This syntax plays horribly with the C preprocessor:
+
+....
+MACRO($'a)
+....
+
+fails because cpp treats string and char literals magically.
+
 === NOP instructions
 
 * x86: link:userland/arch/x86_64/nop.S[NOP]

userland/arch/x86_64/char_literals.S

@@ -0,0 +1,25 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#gnu-gas-assembler-char-literals */
+
+#include <lkmc.h>
+
+LKMC_PROLOGUE
+    mov $0, %r12
+
+    /* Memory. */
+    mov mychar, %r12b
+    LKMC_ASSERT_EQ(%r12, $0x61)
+
+    /* Immediate. Without the `$`, does a memory access, and segfaults! */
+    mov $'b, %r12b
+    LKMC_ASSERT_EQ(%r12, $0x62)
+
+    /* Space character works. */
+    mov $' , %r12b
+    LKMC_ASSERT_EQ(%r12, $0x20)
+
+    /* Backslash escapes work. */
+    mov $'\n , %r12b
+    LKMC_ASSERT_EQ(%r12, $0x0A)
+LKMC_EPILOGUE
+mychar:
+    .byte 'a