diff options
| author | Eli Friedman <eli.friedman@gmail.com> | 2008-11-30 07:52:27 +0000 |
|---|---|---|
| committer | Eli Friedman <eli.friedman@gmail.com> | 2008-11-30 07:52:27 +0000 |
| commit | 91db5271d2f09e4b56e3658277417408ffa70cbe (patch) | |
| tree | 664611c6e5956f6fa5d20e4a800bbf879a51c409 /lib | |
| parent | 4e16b29a6b1830b977ac27de5954c1b9ee17984f (diff) | |
A couple small cleanups, plus a new potential optimization.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60286 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'lib')
| -rw-r--r-- | lib/Target/X86/README.txt | 32 |
1 files changed, 29 insertions, 3 deletions
diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt index 9274762345..c02eca6410 100644 --- a/lib/Target/X86/README.txt +++ b/lib/Target/X86/README.txt @@ -394,6 +394,10 @@ estimate to determine whether the match is profitable. However, if we care more about code size, then imull is better. It's two bytes shorter than movl + leal. +On a Pentium M, both variants have the same characteristics with regard +to throughput; however, the multiplication has a latency of four cycles, as +opposed to two cycles for the movl+lea variant. + //===---------------------------------------------------------------------===// __builtin_ffs codegen is messy. @@ -628,9 +632,9 @@ imagine there has to be some kind of complicated decoder reset and realignment to grab the bytes from the next cacheline. 532 532 0x3cfc movb (1809(%esp, %esi), %bl <<<--- spans 2 64 byte lines -942 942 0x3d03 movl %dh, (1809(%esp, %esi) -937 937 0x3d0a incl %esi -3 3 0x3d0b cmpb %bl, %dl +942 942 0x3d03 movl %dh, (1809(%esp, %esi) +937 937 0x3d0a incl %esi +3 3 0x3d0b cmpb %bl, %dl 27 27 0x3d0d jnz 0x000062db <main+11707> //===---------------------------------------------------------------------===// @@ -1790,3 +1794,25 @@ def : Pat<(v8i8 (bitconvert (i64 (vector_extract (v2i64 VR128:$src), (v8i8 (MMX_MOVDQ2Qrr VR128:$src))>; There are other cases in various td files. + +//===---------------------------------------------------------------------===// + +Take something like the following on x86-32: +unsigned a(unsigned long long x, unsigned y) {return x % y;} + +We currently generate a libcall, but we really shouldn't: the expansion is +shorter and likely faster than the libcall. The expected code is something +like the following: + + movl 12(%ebp), %eax + movl 16(%ebp), %ecx + xorl %edx, %edx + divl %ecx + movl 8(%ebp), %eax + divl %ecx + movl %edx, %eax + ret + +A similar code sequence works for division. + +//===---------------------------------------------------------------------===// |