From 15230bbb9b1f69def9b0e1b41a097638c0fda734 Mon Sep 17 00:00:00 2001 From: johannst Date: Tue, 28 Apr 2020 09:11:18 +0000 Subject: deploy: fef4d6ff2ad9f48e6dccde0f061453e6a3ac624e --- gcc.html | 261 --------------------------------------------------------------- 1 file changed, 261 deletions(-) delete mode 100644 gcc.html (limited to 'gcc.html') diff --git a/gcc.html b/gcc.html deleted file mode 100644 index 196ab3a..0000000 --- a/gcc.html +++ /dev/null @@ -1,261 +0,0 @@ - - - - - - gcc - Notes - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- -
- - - - - - - - - - -
-
-

gcc(1)

-

CLI

-

Preprocessing

-

While debugging can be helpful to just pre-process files.

-
gcc -E [-dM] ...
-
-
    -
  • -E run only preprocessor
    • -
  • -dM list only #define statements
    • -
-

Builtins

-

__builtin_expect(expr, cond)

-

Give the compiler a hint which branch is hot, so it can lay out the code -accordingly to reduce number of jump instructions. -See on compiler explorer.

-
echo "
-extern void foo();
-extern void bar();
-void run0(int x) {
-  if (__builtin_expect(x,0)) { foo(); }
-  else { bar(); }
-}
-void run1(int x) {
-  if (__builtin_expect(x,1)) { foo(); }
-  else { bar(); }
-}
-" | gcc -O2 -S -masm=intel -o /dev/stdout -xc -
-
-

Will generate something similar to the following.

-
    -
  • run0: bar is on the path without branch
    • -
  • run1: foo is on the path without branch
    • -
-
run0:
-        test    edi, edi
-        jne     .L4
-        xor     eax, eax
-        jmp     bar
-.L4:
-        xor     eax, eax
-        jmp     foo
-run1:
-        test    edi, edi
-        je      .L6
-        xor     eax, eax
-        jmp     foo
-.L6:
-        xor     eax, eax
-        jmp     bar
-
- -
- - -
-
- - - -
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- cgit v1.2.3