[llvm-dev] LEAQ instruction path

[Craig Topper via llvm-dev]

leaq is referred to has LEA64r in the X86 instruction td files. Isel uses
X86DAGToDAGISel::selectLEAAddr to pattern match address calculations for
lea in the DAG.

This is set up by the pattern for LEA64r here

def LEA64r   : RI<0x8D, MRMSrcMem, (outs GR64:$dst), (ins lea64mem:$src),
                  "lea{q}\t{$src|$dst}, {$dst|$src}",
                  [(set GR64:$dst, lea64addr:$src)], IIC_LEA>;

lea64addr is defined as a complex pattern that should call selectLEAAddr to
check for a match. I believe the list of things in the square brackets
indicate the nodes that could possibly the root of an address calculation
that an lea can encode.

def lea64addr : ComplexPattern<i64, 5, "selectLEAAddr",
                        [add, sub, mul, X86mul_imm, shl, or, frameindex,
                         X86WrapperRIP], []>;


For the case you gave I ran it through the backend on trunk. Here's where
it selects the LEA64r instruction. The root nodes in this case is a
FrameIndex

ISEL: Starting pattern match on root node: t2: i64 = FrameIndex<0>

  Initial Opcode index to 130065
  TypeSwitch[i64] from 130066 to 130107
MatchAddress: X86ISelAddressMode 0x7ffee68ae4f8
Base_Reg nul
 Scale 1
IndexReg nul
 Disp 0
GV nul CP nul
ES nul MCSym nul JT-1 Align0
  Morphed node: t2: i64 = LEA64r TargetFrameIndex:i64<0>,
TargetConstant:i8<1>, Register:i64 %noreg, TargetConstant:i32<0>,
Register:i32 %noreg


I hope this helps.


~Craig

On Wed, Jan 17, 2018 at 6:30 PM, Barbora Murinová via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> I've been trying to teach LLVM that pointers are 128-bit long, which
> segfaults with some seemingly unrelated stacktrace when I try to take an
> address of a variable. Since stack saving and loading seems to work fine, I
> dare to assume the instruction causing problems there is leaq. Now I've
> done a search for leaq of the entire LLVM codebase with no success and I'd
> like to know which DAG nodes and eventually instructions does the last
> store in the following piece of code get translated to before it gets
> emitted.
>
>   %1 = alloca i32, align 4
>   %2 = alloca i32*, align 8
>   store i32 10, i32* %1, align 4
>   store i32* %1, i32** %2, align 8
>
> Thanks,
> Barbora
>
>
>
> --
> ----------------
> Barbora Murinová
> The University of Edinburgh
> SK: +421905718390 <+421%20905%20718%20390>
> UK: +447477833795 <+44%207477%20833795>
>
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>
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