diff --git a/llvm/lib/Target/X86/README.txt b/llvm/lib/Target/X86/README.txt index 7e365203a805..bea5b8dbca4d 100644 --- a/llvm/lib/Target/X86/README.txt +++ b/llvm/lib/Target/X86/README.txt @@ -762,3 +762,71 @@ int f(char *p) { We should inline lrintf and probably other libc functions. //===---------------------------------------------------------------------===// + +Start using the flags more. For example, compile: + +int add_zf(int *x, int y, int a, int b) { + if ((*x += y) == 0) + return a; + else + return b; +} + +to: + addl %esi, (%rdi) + movl %edx, %eax + cmovne %ecx, %eax + ret +instead of: + +_add_zf: + addl (%rdi), %esi + movl %esi, (%rdi) + testl %esi, %esi + cmove %edx, %ecx + movl %ecx, %eax + ret + +and: + +int add_zf(int *x, int y, int a, int b) { + if ((*x + y) < 0) + return a; + else + return b; +} + +to: + +add_zf: + addl (%rdi), %esi + movl %edx, %eax + cmovns %ecx, %eax + ret + +instead of: + +_add_zf: + addl (%rdi), %esi + testl %esi, %esi + cmovs %edx, %ecx + movl %ecx, %eax + ret + +//===---------------------------------------------------------------------===// + +This: +#include <math.h> +int foo(double X) { return isnan(X); } + +compiles to (-m64): + +_foo: + pxor %xmm1, %xmm1 + ucomisd %xmm1, %xmm0 + setp %al + movzbl %al, %eax + ret + +the pxor is not needed, we could compare the value against itself. +