[LLVMbugs] [Bug 21297] New: [ARM] Allow selection of the SMULW[BT] and SMLAW[BT] instructions

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=21297

            Bug ID: 21297
           Summary: [ARM] Allow selection of the SMULW[BT] and SMLAW[BT]
                    instructions
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: ARM
          Assignee: unassignedbugs at nondot.org
          Reporter: oliver.stannard at arm.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Our current instruction selection for the SMULW[BT] and SMLAW[BT] instructions
is incorrect (http://llvm.org/bugs/show_bug.cgi?id=19396), and I have a patch
up for review to remove the incorrect selection patterns
(http://reviews.llvm.org/D5824).

However, it should be possible to use these instructions from C code, for
example:

int smulwb(int a, short b) {
  return ((long long)a * (long long)b) >> 16;
}

This involves a 64-bit multiply on a 32-bit target, which is an isel DAG node
with two results, so it is not possible to write a pattern to select this. This
would require C++ code to perform the match.

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