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| author | Reid Barton <rwbarton@gmail.com> | 2017-03-23 21:02:29 -0400 |
|---|---|---|
| committer | Ben Gamari <ben@smart-cactus.org> | 2017-03-24 10:14:26 -0400 |
| commit | caf94b062a0e37ffa7048e51447fc9486b658917 (patch) | |
| tree | d14fdcbcbdf3844ae80dbe423396d825974bbe4e /compiler/nativeGen | |
| parent | 8429a202ec8aef0af226d3fb5907b47dc9cba155 (diff) | |
| download | haskell-caf94b062a0e37ffa7048e51447fc9486b658917.tar.gz | |
x86 nativeGen: Fix test with mask in range [128,255] (#13425)
My commit bdb0c43c7 optimized the encoding of instructions to test
tag bits, but it did not always set exactly the same condition codes
since the testb instruction does a single-byte comparison, rather
than a full-word comparison.
It would be correct to optimize the expression `x .&. 128 > 0` to
the sequence
testb $128, %al
seta %al ; note: 'a' for unsigned comparison,
; not 'g' for signed comparison
but the pretty-printer is not the right place to make this kind of
context-sensitive optimization.
Test Plan: harbormaster
Reviewers: trofi, austin, bgamari, dfeuer
Reviewed By: trofi, dfeuer
Subscribers: thomie
Differential Revision: https://phabricator.haskell.org/D3359
Diffstat (limited to 'compiler/nativeGen')
| -rw-r--r-- | compiler/nativeGen/X86/Ppr.hs | 6 |
1 files changed, 5 insertions, 1 deletions
diff --git a/compiler/nativeGen/X86/Ppr.hs b/compiler/nativeGen/X86/Ppr.hs index 7d19e990ce..5044c83c89 100644 --- a/compiler/nativeGen/X86/Ppr.hs +++ b/compiler/nativeGen/X86/Ppr.hs @@ -671,8 +671,12 @@ pprInstr (TEST format src dst) = sdocWithPlatform $ \platform -> -- (We could handle masks larger than a single byte too, -- but it would complicate the code considerably -- and tag checks are by far the most common case.) + -- The mask must have the high bit clear for this smaller encoding + -- to be completely equivalent to the original; in particular so + -- that the signed comparison condition bits are the same as they + -- would be if doing a full word comparison. See Trac #13425. (OpImm (ImmInteger mask), OpReg dstReg) - | 0 <= mask && mask < 256 -> minSizeOfReg platform dstReg + | 0 <= mask && mask < 128 -> minSizeOfReg platform dstReg _ -> format in pprFormatOpOp (sLit "test") format' src dst where |