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A couple small cleanups, plus a new potential optimization.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60286 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Eli Friedman 2008-11-30 07:52:27 +00:00
parent 4e16b29a6b
commit 91db5271d2
1 changed files with 29 additions and 3 deletions
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32
lib/Target/X86/README.txt
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@ -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.
//===---------------------------------------------------------------------===//