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Add a note about integer multiplication by constants.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28551 91177308-0d34-0410-b5e6-96231b3b80d8
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Evan Cheng 2006-05-30 07:37:37 +00:00
parent e6ad27e917
commit 5a622f2e06
1 changed files with 27 additions and 0 deletions
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lib/Target/X86/README.txt
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@ -622,3 +622,30 @@ How about implementing truncate / anyext as a property of machine instruction
operand? i.e. Print as 32-bit super-class register / 16-bit sub-class register.
Do this for the cases where a truncate / anyext is guaranteed to be eliminated.
For IA32 that is truncate from 32 to 16 and anyext from 16 to 32.
//===---------------------------------------------------------------------===//
For this:
int test(int a)
{
return a * 3;
}
We currently emits
imull $3, 4(%esp), %eax
Perhaps this is what we really should generate is? Is imull three or four
cycles? Note: ICC generates this:
movl 4(%esp), %eax
leal (%eax,%eax,2), %eax
The current instruction priority is based on pattern complexity. The former is
more "complex" because it folds a load so the latter will not be emitted.
Perhaps we should use AddedComplexity to give LEA32r a higher priority? We
should always try to match LEA first since the LEA matching code does some
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.