[ARM] Check for correct HW div when lowering divmod

[rovka]

Summary

For subtargets that use the custom lowering for divmod, e.g. gnueabi,
we used to check if the subtarget has hardware divide and then lower to
a div-mul-sub sequence if true, or to a libcall if false.

However, judging by the usage of hasDivide vs hasDivideInARMMode, it
seems that hasDivide only refers to Thumb. For instance, in the
ARMTargetLowering constructor, the code that specifies whether to use
libcalls for (S|U)DIV looks like this:

bool hasDivide = Subtarget->isThumb() ? Subtarget->hasDivide()
                                      : Subtarget->hasDivideInARMMode();

In the case of divmod for arm-gnueabi, using only hasDivide() to
determine what to do means that instead of lowering to _aeabi_idivmod
to get the remainder, we lower to div-mul-sub and then further lower the
div to _aeabi_idiv. Even worse, if we have hardware divide in ARM but
not in Thumb, we generate a libcall instead of using it (this is not an
issue in practice since AFAICT none of the cores that we support have
hardware divide in ARM but not Thumb).

This patch fixes the code dealing with custom lowering to take into
account the mode (Thumb or ARM) when deciding whether or not hardware
division is available.

[compnerd]

Minor comments on the test, but LGTM otherwise.

test/CodeGen/ARM/divmod-hwdiv.ll

7 ↗

(On Diff #95101)

I'm not sure I understand what the duplication here is for. The compilation mode is identical to the ones above. We should remove these I believe.

[rovka]

Thanks for the review

test/CodeGen/ARM/divmod-hwdiv.ll

7 ↗

(On Diff #95101)

The difference is hwdiv (hardware divide in thumb) vs hwdiv-arm (hardware divide in arm). The first 4 lines check that hwdiv influences thumb, but not arm, and the last 4 check that hwdiv-arm influences arm, but not thumb.

llvm/trunk/lib/Target/ARM/ARMISelLowering.cpp

SDValue ARMTargetLowering::LowerDivRem(SDValue Op, SelectionDAG &DAG) const {
   Type *Ty = VT.getTypeForEVT(*DAG.getContext());
   SDLoc dl(Op);
 
   // If the target has hardware divide, use divide + multiply + subtract:
   //     div = a / b
   //     rem = a - b * div
   //     return {div, rem}
   // This should be lowered into UDIV/SDIV + MLS later on.
-  if (Subtarget->hasDivide() && Op->getValueType(0).isSimple() &&
+  bool hasDivide = Subtarget->isThumb() ? Subtarget->hasDivide()
+                                        : Subtarget->hasDivideInARMMode();
+  if (hasDivide && Op->getValueType(0).isSimple() &&
       Op->getSimpleValueType(0) == MVT::i32) {
     unsigned DivOpcode = isSigned ? ISD::SDIV : ISD::UDIV;
     const SDValue Dividend = Op->getOperand(0);
     const SDValue Divisor = Op->getOperand(1);
     SDValue Div = DAG.getNode(DivOpcode, dl, VT, Dividend, Divisor);
     SDValue Mul = DAG.getNode(ISD::MUL, dl, VT, Div, Divisor);
     SDValue Rem = DAG.getNode(ISD::SUB, dl, VT, Dividend, Mul);
 

llvm/trunk/test/CodeGen/ARM/divmod-hwdiv.ll

+; The hwdiv subtarget feature should only influence thumb, not arm.
+; RUN: llc < %s -mtriple=arm-gnueabi -mattr=+hwdiv | FileCheck %s -check-prefixes=ALL,AEABI-NOHWDIV
+; RUN: llc < %s -mtriple=arm-gnueabi -mattr=-hwdiv | FileCheck %s -check-prefixes=ALL,AEABI-NOHWDIV
+; RUN: llc < %s -mtriple=thumbv7-gnueabi -mattr=+hwdiv | FileCheck %s -check-prefixes=ALL,THUMB-HWDIV
+; RUN: llc < %s -mtriple=thumbv7-gnueabi -mattr=-hwdiv | FileCheck %s -check-prefixes=ALL,AEABI-NOHWDIV
+
+; The hwdiv-arm subtarget feature should only influence arm, not thumb.
+; RUN: llc < %s -mtriple=arm-gnueabi -mattr=+hwdiv-arm | FileCheck %s -check-prefixes=ALL,ARM-HWDIV
+; RUN: llc < %s -mtriple=arm-gnueabi -mattr=-hwdiv-arm | FileCheck %s -check-prefixes=ALL,AEABI-NOHWDIV
+; RUN: llc < %s -mtriple=thumbv7-gnueabi -mattr=+hwdiv-arm | FileCheck %s -check-prefixes=ALL,AEABI-NOHWDIV
+; RUN: llc < %s -mtriple=thumbv7-gnueabi -mattr=-hwdiv-arm | FileCheck %s -check-prefixes=ALL,AEABI-NOHWDIV
+
+define arm_aapcscc i32 @test_i32_srem(i32 %x, i32 %y) {
+; ALL-LABEL: test_i32_srem:
+; ARM-HWDIV: sdiv [[Q:r[0-9]+]], r0, r1
+; ARM-HWDIV: mul [[P:r[0-9]+]], [[Q]], r1
+; ARM-HWDIV: sub r0, r0, [[P]]
+; THUMB-HWDIV: sdiv [[Q:r[0-9]+]], r0, r1
+; THUMB-HWDIV: mls r0, [[Q]], r1, r0
+; AEABI-NOHWDIV: bl __aeabi_idivmod
+; AEABI-NOHWDIV: mov r0, r1
+  %r = srem i32 %x, %y
+  ret i32 %r
+}
+
+define arm_aapcscc i32 @test_i32_urem(i32 %x, i32 %y) {
+; ALL-LABEL: test_i32_urem:
+; ARM-HWDIV: udiv [[Q:r[0-9]+]], r0, r1
+; ARM-HWDIV: mul [[P:r[0-9]+]], [[Q]], r1
+; ARM-HWDIV: sub r0, r0, [[P]]
+; THUMB-HWDIV: udiv [[Q:r[0-9]+]], r0, r1
+; THUMB-HWDIV: mls r0, [[Q]], r1, r0
+; AEABI-NOHWDIV: bl __aeabi_uidivmod
+; AEABI-NOHWDIV: mov r0, r1
+  %r = urem i32 %x, %y
+  ret i32 %r
+}