diff options
| -rw-r--r-- | lib/Target/ARM/README.txt | 16 |
1 files changed, 16 insertions, 0 deletions
diff --git a/lib/Target/ARM/README.txt b/lib/Target/ARM/README.txt index 2f6842e8cb..4fcaecfcb2 100644 --- a/lib/Target/ARM/README.txt +++ b/lib/Target/ARM/README.txt @@ -699,3 +699,19 @@ test is equality test so it's more a conditional move rather than a select: Currently this is a ARM specific dag combine. We probably should make it into a target-neutral one. + +//===---------------------------------------------------------------------===// + +Optimize unnecessary checks for zero with __builtin_clz/ctz. Those builtins +are specified to be undefined at zero, so portable code must check for zero +and handle it as a special case. That is unnecessary on ARM where those +operations are implemented in a way that is well-defined for zero. For +example: + +int f(int x) { return x ? __builtin_clz(x) : sizeof(int)*8; } + +should just be implemented with a CLZ instruction. Since there are other +targets, e.g., PPC, that share this behavior, it would be best to implement +this in a target-independent way: we should probably fold that (when using +"undefined at zero" semantics) to set the "defined at zero" bit and have +the code generator expand out the right code. |