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Yet more readme updating

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28172 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Nate Begeman 2006-05-08 20:54:02 +00:00
parent a850446a0b
commit 7514620052
1 changed files with 22 additions and 36 deletions
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58
lib/Target/PowerPC/README.txt
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@ -492,18 +492,23 @@ transformation, good for PI. See PPCISelLowering.cpp, this comment:
===-------------------------------------------------------------------------===
void %foo(uint *%tmp) {
%tmp = load uint* %tmp ; <uint> [#uses=3]
%tmp1 = shr uint %tmp, ubyte 31 ; <uint> [#uses=1]
%tmp1 = cast uint %tmp1 to ubyte ; <ubyte> [#uses=1]
%tmp4.mask = shr uint %tmp, ubyte 30 ; <uint> [#uses=1]
%tmp4.mask = cast uint %tmp4.mask to ubyte ; <ubyte> [#uses=1]
%tmp = or ubyte %tmp4.mask, %tmp1 ; <ubyte> [#uses=1]
%tmp10 = cast ubyte %tmp to uint ; <uint> [#uses=1]
%tmp11 = shl uint %tmp10, ubyte 31 ; <uint> [#uses=1]
%tmp12 = and uint %tmp, 2147483647 ; <uint> [#uses=1]
%tmp13 = or uint %tmp11, %tmp12 ; <uint> [#uses=1]
store uint %tmp13, uint* %tmp
%struct.B = type { ubyte, [3 x ubyte] }
void %foo(%struct.B* %b) {
entry:
%tmp = cast %struct.B* %b to uint* ; <uint*> [#uses=1]
%tmp = load uint* %tmp ; <uint> [#uses=1]
%tmp3 = cast %struct.B* %b to uint* ; <uint*> [#uses=1]
%tmp4 = load uint* %tmp3 ; <uint> [#uses=1]
%tmp8 = cast %struct.B* %b to uint* ; <uint*> [#uses=2]
%tmp9 = load uint* %tmp8 ; <uint> [#uses=1]
%tmp4.mask17 = shl uint %tmp4, ubyte 1 ; <uint> [#uses=1]
%tmp1415 = and uint %tmp4.mask17, 2147483648 ; <uint> [#uses=1]
%tmp.masked = and uint %tmp, 2147483648 ; <uint> [#uses=1]
%tmp11 = or uint %tmp1415, %tmp.masked ; <uint> [#uses=1]
%tmp12 = and uint %tmp9, 2147483647 ; <uint> [#uses=1]
%tmp13 = or uint %tmp12, %tmp11 ; <uint> [#uses=1]
store uint %tmp13, uint* %tmp8
ret void
}
@ -511,15 +516,14 @@ We emit:
_foo:
lwz r2, 0(r3)
srwi r4, r2, 30
srwi r5, r2, 31
or r4, r4, r5
rlwimi r2, r4, 31, 0, 0
slwi r4, r2, 1
or r4, r4, r2
rlwimi r2, r4, 0, 0, 0
stw r2, 0(r3)
blr
What this code is really doing is ORing bit 0 with bit 1. We could codegen this
as:
We could collapse a bunch of those ORs and ANDs and generate the following
equivalent code:
_foo:
lwz r2, 0(r3)
@ -527,21 +531,3 @@ _foo:
or r2, r2, r4
stw r2, 0(r3)
blr
===-------------------------------------------------------------------------===
Distilled from the code above, something wacky is going in the optimizers before
code generation time...
unsigned foo(unsigned x) {
return (unsigned)((unsigned char)(x >> 30) | (unsigned char)(x >> 31)) << 31;
}
unsigned bar(unsigned x) {
return ((x >> 30) | (x >> 31)) << 31;
}
generate different code when -O is passed to llvm-gcc. However, when no
optimization is specified and the output is passed into opt with just -mem2reg
and -instcombine, the good code comes out of both. Something is happening before
instcombine to confuse it, and not delete the no-op casts.