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