http://llvm.org/bugs/show_bug.cgi?id=12212
Bug #: 12212
Summary: LLVM omits the multiplication-to-shift conversion for
negative non-power-of 2 constants
Product: new-bugs
Version: unspecified
Platform: PC
OS/Version: All
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
Classification: Unclassified
On ARM architecture, LLVM is missing multiplication-to-shift conversions when
one operand is a negative constant and the other operand is an integer typed
variable.
For example, for
foo(x)
{
return x * -3;
}
LLVM emits a "mul" instr.
However, it should be converted to x-x<<2 (sub r0, r0, r0, lsl #2), which is a
more efficient operation on ARM than the "mul" instruction.
For positive constants, like x*3, LLVM emits the shift/add instruction
correctly.
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