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6f3828aa04
Add generic instructions for load complement, load negative and load positive for fp32 and fp64, and let isel prefer them. They do not clobber CC, and so give scheduler more freedom. SystemZElimCompare pass will convert them when it can to the CC-setting variants. Regression tests updated to expect the new opcodes in places where the old ones where used. New test case SystemZ/fp-cmp-05.ll checks that SystemZCompareElim.cpp can handle the new opcodes. README.txt updated (bullet removed). Note that fp128 is not yet handled, because it is relatively rare, and is a bit trickier, because of the fact that l.dfr would operate on the sign bit of one of the subregisters of a fp128, but we would not want to copy the other sub-reg in case src and dst regs are not the same. Reviewed by Ulrich Weigand. llvm-svn: 249046
169 lines
4.1 KiB
Plaintext
169 lines
4.1 KiB
Plaintext
//===---------------------------------------------------------------------===//
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// Random notes about and ideas for the SystemZ backend.
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//===---------------------------------------------------------------------===//
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The initial backend is deliberately restricted to z10. We should add support
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for later architectures at some point.
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--
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SystemZDAGToDAGISel::SelectInlineAsmMemoryOperand() is passed "m" for all
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inline asm memory constraints; it doesn't get to see the original constraint.
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This means that it must conservatively treat all inline asm constraints
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as the most restricted type, "R".
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--
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If an inline asm ties an i32 "r" result to an i64 input, the input
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will be treated as an i32, leaving the upper bits uninitialised.
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For example:
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define void @f4(i32 *%dst) {
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%val = call i32 asm "blah $0", "=r,0" (i64 103)
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store i32 %val, i32 *%dst
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ret void
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}
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from CodeGen/SystemZ/asm-09.ll will use LHI rather than LGHI.
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to load 103. This seems to be a general target-independent problem.
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--
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The tuning of the choice between LOAD ADDRESS (LA) and addition in
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SystemZISelDAGToDAG.cpp is suspect. It should be tweaked based on
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performance measurements.
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--
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There is no scheduling support.
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--
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We don't use the BRANCH ON INDEX instructions.
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--
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We might want to use BRANCH ON CONDITION for conditional indirect calls
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and conditional returns.
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--
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We don't use the TEST DATA CLASS instructions.
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--
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We only use MVC, XC and CLC for constant-length block operations.
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We could extend them to variable-length operations too,
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using EXECUTE RELATIVE LONG.
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MVCIN, MVCLE and CLCLE may be worthwhile too.
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--
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We don't use CUSE or the TRANSLATE family of instructions for string
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operations. The TRANSLATE ones are probably more difficult to exploit.
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--
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We don't take full advantage of builtins like fabsl because the calling
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conventions require f128s to be returned by invisible reference.
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--
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ADD LOGICAL WITH SIGNED IMMEDIATE could be useful when we need to
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produce a carry. SUBTRACT LOGICAL IMMEDIATE could be useful when we
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need to produce a borrow. (Note that there are no memory forms of
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ADD LOGICAL WITH CARRY and SUBTRACT LOGICAL WITH BORROW, so the high
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part of 128-bit memory operations would probably need to be done
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via a register.)
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--
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We don't use the halfword forms of LOAD REVERSED and STORE REVERSED
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(LRVH and STRVH).
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--
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We don't use ICM or STCM.
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--
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DAGCombiner doesn't yet fold truncations of extended loads. Functions like:
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unsigned long f (unsigned long x, unsigned short *y)
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{
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return (x << 32) | *y;
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}
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therefore end up as:
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sllg %r2, %r2, 32
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llgh %r0, 0(%r3)
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lr %r2, %r0
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br %r14
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but truncating the load would give:
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sllg %r2, %r2, 32
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lh %r2, 0(%r3)
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br %r14
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--
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Functions like:
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define i64 @f1(i64 %a) {
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%and = and i64 %a, 1
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ret i64 %and
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}
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ought to be implemented as:
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lhi %r0, 1
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ngr %r2, %r0
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br %r14
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but two-address optimisations reverse the order of the AND and force:
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lhi %r0, 1
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ngr %r0, %r2
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lgr %r2, %r0
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br %r14
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CodeGen/SystemZ/and-04.ll has several examples of this.
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--
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Out-of-range displacements are usually handled by loading the full
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address into a register. In many cases it would be better to create
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an anchor point instead. E.g. for:
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define void @f4a(i128 *%aptr, i64 %base) {
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%addr = add i64 %base, 524288
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%bptr = inttoptr i64 %addr to i128 *
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%a = load volatile i128 *%aptr
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%b = load i128 *%bptr
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%add = add i128 %a, %b
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store i128 %add, i128 *%aptr
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ret void
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}
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(from CodeGen/SystemZ/int-add-08.ll) we load %base+524288 and %base+524296
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into separate registers, rather than using %base+524288 as a base for both.
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--
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Dynamic stack allocations round the size to 8 bytes and then allocate
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that rounded amount. It would be simpler to subtract the unrounded
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size from the copy of the stack pointer and then align the result.
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See CodeGen/SystemZ/alloca-01.ll for an example.
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--
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If needed, we can support 16-byte atomics using LPQ, STPQ and CSDG.
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--
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We might want to model all access registers and use them to spill
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32-bit values.
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