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cmd/compile: fix buggy AMD64 rewrite from CL 213058
CL 213058's "bonus optimization I noticed while working on this" turns out to be buggy. It would be correct for CMP, but not TEST. Fix it to use TEST semantics instead. This was breaking compilation with the upcoming Spectre mode. Change-Id: If2d4c3798ed182f35f0244febe74e68c61e4c61b Reviewed-on: https://go-review.googlesource.com/c/go/+/222853 Reviewed-by: Keith Randall <[email protected]>
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Lines changed: 52 additions & 3 deletions

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‎src/cmd/compile/internal/ssa/gen/AMD64.rules‎

Lines changed: 4 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1254,7 +1254,10 @@
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(CMPWconst (ANDLconst _ [m]) [n]) && 0 <= int16(m) && int16(m) < int16(n) -> (FlagLT_ULT)
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(CMPBconst (ANDLconst _ [m]) [n]) && 0 <= int8(m) && int8(m) < int8(n) -> (FlagLT_ULT)
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1257-
(TEST(Q|L)const [c] (MOV(Q|L)const [c])) -> (FlagEQ)
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// TESTQ c c sets flags like CMPQ c 0.
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(TEST(Q|L)const [c] (MOV(Q|L)const [c])) && c == 0 -> (FlagEQ)
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(TEST(Q|L)const [c] (MOV(Q|L)const [c])) && c < 0 -> (FlagLT_UGT)
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(TEST(Q|L)const [c] (MOV(Q|L)const [c])) && c > 0 -> (FlagGT_UGT)
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// TODO: DIVxU also.
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‎src/cmd/compile/internal/ssa/rewriteAMD64.go‎

Lines changed: 48 additions & 2 deletions
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