https://lore.kernel.org/llvm/20220210223134.233757-1-morbo@google.com/ improvements

clang.diff

--- slab.before.clang.txt       2022-02-14 14:40:46.324688583 -0800
+++ slab.after.clang.txt        2022-02-14 14:40:27.824562293 -0800
@@ -8,61 +8,51 @@
        push   %r15
        push   %r14
        push   %rbx
-       sub    $0x10,%rsp
-       mov    %gs:0x28,%rax
-       mov    %rax,0x8(%rsp)
        test   %rdi,%rdi
-       je     <kmem_cache_free+0x55>
+       je     <kmem_cache_free+0x36>
        mov    %rsi,%r14
        mov    %rdi,%rbx
-       mov    0x28(%rsp),%rsi
+       mov    0x18(%rsp),%rsi
        mov    0x58(%rdi),%rcx
        xchg   %ax,%ax
-       movq   $0x0,(%rsp)
        pushf  
-       pop    (%rsp)
-       mov    (%rsp),%r15
+       pop    %r15
        cli    
        xchg   %ax,%ax
        mov    %rbx,%rdi
        mov    %r14,%rsi
-       call   <kmem_cache_free+0x4b>
+       call   <kmem_cache_free+0x2c>
        test   $0x200,%r15d
-       je     <kmem_cache_free+0x55>
+       je     <kmem_cache_free+0x36>
        sti    
-       mov    %gs:0x28,%rax
-       cmp    0x8(%rsp),%rax
-       jne    <kmem_cache_free+0x6f>
-       add    $0x10,%rsp
        pop    %rbx
        pop    %r14
        pop    %r15
        ret    
-       call   <kmem_cache_free+0x74>
-       mov    %gs:0x0(%rip),%eax        # <kmem_cache_free+0x7b>
+       mov    %gs:0x0(%rip),%eax        # <kmem_cache_free+0x43>
        mov    %eax,%eax
-       bt     %rax,0x0(%rip)        # <kmem_cache_free+0x85>
-       jae    <kmem_cache_free+0x2d>
-       incl   %gs:0x0(%rip)        # <kmem_cache_free+0x8e>
-       mov    0x0(%rip),%rax        # <kmem_cache_free+0x95>
+       bt     %rax,0x0(%rip)        # <kmem_cache_free+0x4d>
+       jae    <kmem_cache_free+0x1b>
+       incl   %gs:0x0(%rip)        # <kmem_cache_free+0x56>
+       mov    0x0(%rip),%rax        # <kmem_cache_free+0x5d>
        test   %rax,%rax
-       je     <kmem_cache_free+0xa6>
+       je     <kmem_cache_free+0x6e>
        mov    0x8(%rax),%rdi
        mov    %r14,%rdx
-       call   <kmem_cache_free+0xa6>
-       decl   %gs:0x0(%rip)        # <kmem_cache_free+0xad>
-       jne    <kmem_cache_free+0x2d>
-       call   <kmem_cache_free+0xb8>
-       jmp    <kmem_cache_free+0x2d>
+       call   <kmem_cache_free+0x6e>
+       decl   %gs:0x0(%rip)        # <kmem_cache_free+0x75>
+       jne    <kmem_cache_free+0x1b>
+       call   <kmem_cache_free+0x7c>
+       jmp    <kmem_cache_free+0x1b>
        cmpq   $0x0,0x50(%rbx)
-       jne    <kmem_cache_free+0x40>
+       jne    <kmem_cache_free+0x21>
        testl  $0x80800,0x20(%rbx)
-       jne    <kmem_cache_free+0x40>
+       jne    <kmem_cache_free+0x21>
        movslq 0x74(%rbx),%rdx
        mov    %r14,%rdi
        xor    %esi,%esi
-       call   <kmem_cache_free+0xe3>
-       jmp    <kmem_cache_free+0x40>
+       call   <kmem_cache_free+0x9c>
+       jmp    <kmem_cache_free+0x21>
 
 Disassembly of section .init.text:

test.sh

# test clang before + after
make LLVM=1 -j72 defconfig

<manually enable CONFIG_SLAB=y via menuconfig>

make LLVM=1 -j72 mm/slab.o

objdump --no-show-raw-insn --no-addresses mm/slab.o --disassemble=kmem_cache_free > slab.before.clang.txt

b4 am https://lore.kernel.org/llvm/20220210223134.233757-1-morbo@google.com/ -o - | git am

make LLVM=1 -j72 mm/slab.o

objdump --no-show-raw-insn --no-addresses mm/slab.o --disassemble=kmem_cache_free > slab.after.clang.txt

make -j72 defconfig

<manually enable CONFIG_SLAB=y via menuconfig>

make -j72 mm/slab.o

objdump --no-show-raw-insn --no-addresses mm/slab.o --disassemble=kmem_cache_free > slab.after.gcc.txt

git reset --hard HEAD~

# no output
diff -u slab.before.gcc.txt slab.after.gcc.txt