diff options
| -rw-r--r-- | lib/Target/X86/README.txt | 28 |
1 files changed, 28 insertions, 0 deletions
diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt index 9ab17dc3ee..477838d0e5 100644 --- a/lib/Target/X86/README.txt +++ b/lib/Target/X86/README.txt @@ -1094,5 +1094,33 @@ it should compile to a move from the stack slot directly into eax. DAGCombine has this xform, but it is currently disabled until the alignment fields of the load/store nodes are trustworthy. +//===---------------------------------------------------------------------===// + +Sometimes it is better to codegen subtractions from a constant (e.g. 7-x) with +a neg instead of a sub instruction. Consider: + +int test(char X) { return 7-X; } + +we currently produce: +_test: + movl $7, %eax + movsbl 4(%esp), %ecx + subl %ecx, %eax + ret +We would use one fewer register if codegen'd as: + + movsbl 4(%esp), %eax + neg %eax + add $7, %eax + ret + +Note that this isn't beneficial if the load can be folded into the sub. In +this case, we want a sub: + +int test(int X) { return 7-X; } +_test: + movl $7, %eax + subl 4(%esp), %eax + ret |