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add a note: we need whole-function selectiondags :) 

llvm-svn: 33252
This commit is contained in:
Chris Lattner 2007-01-16 06:39:48 +00:00
parent 4dc4489286
commit 89e5813312
1 changed files with 28 additions and 0 deletions
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28
llvm/lib/Target/README.txt
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@ -387,3 +387,31 @@ void good(unsigned x)
to enable further optimizations.
//===---------------------------------------------------------------------===//
Consider:
typedef unsigned U32;
typedef unsigned long long U64;
int test (U32 *inst, U64 *regs) {
U64 effective_addr2;
U32 temp = *inst;
int r1 = (temp >> 20) & 0xf;
int b2 = (temp >> 16) & 0xf;
effective_addr2 = temp & 0xfff;
if (b2) effective_addr2 += regs[b2];
b2 = (temp >> 12) & 0xf;
if (b2) effective_addr2 += regs[b2];
effective_addr2 &= regs[4];
if ((effective_addr2 & 3) == 0)
return 1;
return 0;
}
Note that only the low 2 bits of effective_addr2 are used. On 32-bit systems,
we don't eliminate the computation of the top half of effective_addr2 because
we don't have whole-function selection dags. On x86, this means we use one
extra register for the function when effective_addr2 is declared as U64 than
when it is declared U32.
//===---------------------------------------------------------------------===//