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add a shift/imul missed optimization

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121850 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2010-12-15 07:10:43 +00:00
parent 2fc36e19b1
commit ef17f08dbc
1 changed files with 45 additions and 0 deletions
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45
lib/Target/README.txt
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@ -810,6 +810,51 @@ badness. PPC64 misses f, f5 and f6. CellSPU aborts in isel.
//===---------------------------------------------------------------------===//
This (and similar related idioms):
unsigned int foo(unsigned char i) {
return i | (i<<8) | (i<<16) | (i<<24);
}
compiles into:
define i32 @foo(i8 zeroext %i) nounwind readnone ssp noredzone {
entry:
%conv = zext i8 %i to i32
%shl = shl i32 %conv, 8
%shl5 = shl i32 %conv, 16
%shl9 = shl i32 %conv, 24
%or = or i32 %shl9, %conv
%or6 = or i32 %or, %shl5
%or10 = or i32 %or6, %shl
ret i32 %or10
}
it would be better as:
unsigned int bar(unsigned char i) {
unsigned int j=i | (i << 8);
return j | (j<<16);
}
aka:
define i32 @bar(i8 zeroext %i) nounwind readnone ssp noredzone {
entry:
%conv = zext i8 %i to i32
%shl = shl i32 %conv, 8
%or = or i32 %shl, %conv
%shl5 = shl i32 %or, 16
%or6 = or i32 %shl5, %or
ret i32 %or6
}
or even i*0x01010101, depending on the speed of the multiplier. The best way to
handle this is to canonicalize it to a multiply in IR and have codegen handle
lowering multiplies to shifts on cpus where shifts are faster.
//===---------------------------------------------------------------------===//
We do a number of simplifications in simplify libcalls to strength reduce
standard library functions, but we don't currently merge them together. For
example, it is useful to merge memcpy(a,b,strlen(b)) -> strcpy. This can only