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readme updates 

llvm-svn: 26405
This commit is contained in:
Nate Begeman 2006-02-27 22:08:36 +00:00
parent bc7a3832e8
commit f918ed2e33
1 changed files with 26 additions and 21 deletions
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47
llvm/lib/Target/PowerPC/README.txt
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@ -52,7 +52,7 @@ This occurs in SPASS.
as:
xoris r0,r3,0x1234
cmpwi cr0,r0,0x5678
cmplwi cr0,r0,0x5678
beq cr0,L6
not:
@ -148,30 +148,35 @@ typedef struct program_t {
void AdjustBitfields(program* prog, unsigned int fmt1)
{
unsigned int shift = 0;
unsigned int texCount = 0;
unsigned int i;
for (i = 0; i < 8; i++)
{
prog->array[i].bitfields.field0 = texCount;
prog->array[i].bitfields.field1 = texCount + 1;
prog->array[i].bitfields.field2 = texCount + 2;
prog->array[i].bitfields.field3 = texCount + 3;
texCount += (fmt1 >> shift) & 0x7;
shift += 3;
}
prog->array[0].bitfields.field0 = fmt1;
prog->array[0].bitfields.field1 = fmt1 + 1;
}
In the loop above, the bitfield adds get generated as
(add (shl bitfield, C1), (shl C2, C1)) where C2 is 1, 2 or 3.
We currently generate:
Since the input to the (or and, and) is an (add) rather than a (shl), the shift
doesn't get folded into the rlwimi instruction. We should ideally see through
things like this, rather than forcing llvm to generate the equivalent
_AdjustBitfields:
lwz r2, 0(r3)
addi r5, r4, 1
rlwinm r2, r2, 0, 0, 19
rlwinm r5, r5, 6, 20, 25
rlwimi r2, r4, 0, 26, 31
or r2, r2, r5
stw r2, 0(r3)
blr
(shl (add bitfield, C2), C1) with some kind of mask.
We should teach someone that or (rlwimi, rlwinm) with disjoint masks can be
turned into rlwimi (rlwimi)
The better codegen would be:
_AdjustBitfields:
lwz r0,0(r3)
rlwinm r4,r4,0,0xff
rlwimi r0,r4,0,26,31
addi r4,r4,1
rlwimi r0,r4,6,20,25
stw r0,0(r3)
blr
===-------------------------------------------------------------------------===