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| -rw-r--r-- | lib/Target/X86/README.txt | 32 |
1 files changed, 32 insertions, 0 deletions
diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt index 5a2da2d3d2..bab702e96e 100644 --- a/lib/Target/X86/README.txt +++ b/lib/Target/X86/README.txt @@ -443,3 +443,35 @@ when we can spare a register. It reduces code size. It's not clear whether we should use pxor or xorps / xorpd to clear XMM registers. The choice may depend on subtarget information. We should do some more experiments on different x86 machines. + +//===---------------------------------------------------------------------===// + +Evaluate what the best way to codegen sdiv X, (2^C) is. For X/8, we currently +get this: + +int %test1(int %X) { + %Y = div int %X, 8 + ret int %Y +} + +_test1: + movl 4(%esp), %eax + movl %eax, %ecx + sarl $31, %ecx + shrl $29, %ecx + addl %ecx, %eax + sarl $3, %eax + ret + +GCC knows several different ways to codegen it, one of which is this: + +_test1: + movl 4(%esp), %eax + cmpl $-1, %eax + leal 7(%eax), %ecx + cmovle %ecx, %eax + sarl $3, %eax + ret + +which is probably slower, but it's interesting at least :) + |