diff options
Diffstat (limited to 'lib/Target/X86/README.txt')
| -rw-r--r-- | lib/Target/X86/README.txt | 82 |
1 files changed, 57 insertions, 25 deletions
diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt index c10e170..abd1515 100644 --- a/lib/Target/X86/README.txt +++ b/lib/Target/X86/README.txt @@ -1879,39 +1879,71 @@ _add32carry: //===---------------------------------------------------------------------===// -This: -char t(char c) { - return c/3; +The hot loop of 256.bzip2 contains code that looks a bit like this: + +int foo(char *P, char *Q, int x, int y) { + if (P[0] != Q[0]) + return P[0] < Q[0]; + if (P[1] != Q[1]) + return P[1] < Q[1]; + if (P[2] != Q[2]) + return P[2] < Q[2]; + return P[3] < Q[3]; } -Compiles to: $clang t.c -S -o - -O3 -mkernel -fomit-frame-pointer +In the real code, we get a lot more wrong than this. However, even in this +code we generate: -_t: ## @t - movslq %edi, %rax - imulq $-1431655765, %rax, %rcx ## imm = 0xFFFFFFFFAAAAAAAB - shrq $32, %rcx - addl %ecx, %eax - movl %eax, %ecx - shrl $31, %ecx - shrl %eax - addl %ecx, %eax - movsbl %al, %eax +_foo: ## @foo +## BB#0: ## %entry + movb (%rsi), %al + movb (%rdi), %cl + cmpb %al, %cl + je LBB0_2 +LBB0_1: ## %if.then + cmpb %al, %cl + jmp LBB0_5 +LBB0_2: ## %if.end + movb 1(%rsi), %al + movb 1(%rdi), %cl + cmpb %al, %cl + jne LBB0_1 +## BB#3: ## %if.end38 + movb 2(%rsi), %al + movb 2(%rdi), %cl + cmpb %al, %cl + jne LBB0_1 +## BB#4: ## %if.end60 + movb 3(%rdi), %al + cmpb 3(%rsi), %al +LBB0_5: ## %if.end60 + setl %al + movzbl %al, %eax ret -GCC gets: +Note that we generate jumps to LBB0_1 which does a redundant compare. The +redundant compare also forces the register values to be live, which prevents +folding one of the loads into the compare. In contrast, GCC 4.2 produces: -_t: - movl $86, %eax - imulb %dil - shrw $8, %ax - sarb $7, %dil - subb %dil, %al - movsbl %al,%eax +_foo: + movzbl (%rsi), %eax + cmpb %al, (%rdi) + jne L10 +L12: + movzbl 1(%rsi), %eax + cmpb %al, 1(%rdi) + jne L10 + movzbl 2(%rsi), %eax + cmpb %al, 2(%rdi) + jne L10 + movzbl 3(%rdi), %eax + cmpb 3(%rsi), %al +L10: + setl %al + movzbl %al, %eax ret -which is nicer. This also happens for int, not just char. +which is "perfect". //===---------------------------------------------------------------------===// - - |
