x86 asm: move loop from x86-assembly-cheat

Author: cirosantilli

README.adoc

@@ -12410,6 +12410,12 @@ JG vs JA and JL vs JB:
 * https://stackoverflow.com/questions/9617877/assembly-jg-jnle-jl-jnge-after-cmp/56613928#56613928
 * https://stackoverflow.com/questions/20906639/difference-between-ja-and-jg-in-assembly
 
+==== x86 LOOP instruction
+
+link:userland/arch/x86_64/loop.S[LOOP]
+
+Vs <<x86-jcc-instructions,Jcc>>: https://stackoverflow.com/questions/6805692/x86-assembly-programming-loops-with-ecx-and-loop-instruction-versus-jmp-jcond Holy CISC!
+
 === x86 SIMD
 
 History:

userland/arch/x86_64/loop.S

@@ -0,0 +1,53 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#x86-loop-instruction */
+
+#include <lkmc.h>
+
+LKMC_PROLOGUE
+
+    /* LOOP
+     *
+     * ....
+     * rcx--;
+     * if (rcx != 0) goto label
+     * ....
+     */
+    mov $0, %rax
+    mov $3, %rcx
+loop_label:
+    inc %rax
+    loop loop_label
+    LKMC_ASSERT_EQ(%rax, $3)
+
+    /* LOOPE
+     *
+     * ....
+     * rcx--;
+     * if (ecx != 0 && ZF == 1) goto label
+     * ....
+     *
+     * Application: search for first non-zero element in a range.
+     *
+     * If found, rax will contain the element index.
+     *
+     * Otherwise, rax contains length + 1.
+     */
+.section .rodata
+    loope_array: .byte 0, 0, 1, 0
+.text
+    /* Array length. */
+    mov $4, %rcx
+    mov $-1, %rax
+loope_label:
+    inc %rax
+    cmpb $0, loope_array(%rax)
+    loope loope_label
+    /* The first non-zero item (1) was at index 2. */
+    LKMC_ASSERT_EQ(%rax, $2)
+
+    /* LOOPNE
+     *
+     * ....
+     * ecx--; if (ecx != 0 && ZF == 0) goto lbl
+     * ....
+     */
+LKMC_EPILOGUE