[LegalizerHelper] Bug in insert position iterator #159716
Description
For my custom backend, I've following GMIR input to legalizer:
%1:_(s32) = COPY $r8
%0:_(s16) = G_TRUNC %1:_(s32)
%2:_(s16), %3:_ = G_SDIVREM %0:_, %0:_
%4:_(s16) = G_INTRINSIC intrinsic(@llvm.custom.fma), %2:_(s16), %3:_(s16), %2:_(s16)
%7:_(s32) = G_ANYEXT %4:_(s16)
$r8 = COPY %7:_(s32)
I've following legalization rule for G_SDIVREM:
getActionDefinitionsBuilder(
{G_UDIVREM, G_SDIVREM})
.clampScalar(0, s32, s64)
.scalarize(0)
.customFor({s32, s64});
Therefore, legalizer tries to widen G_SDIVREM operation to s32. Then, it inserts G_TRUNC to truncate %2, %3 so they can be consumed by the following llvm.custom.fma intrinsic.
The problem occurs with placement of G_TRUNC operations in LegalizerHelp.cpp. Here's the existing logic:
case TargetOpcode::G_SDIVREM:
Observer.changingInstr(MI);
widenScalarSrc(MI, WideTy, 2, TargetOpcode::G_SEXT);
widenScalarSrc(MI, WideTy, 3, TargetOpcode::G_SEXT);
widenScalarDst(MI, WideTy);
//MIRBuilder.setInsertPt(MIRBuilder.getMBB(), --MIRBuilder.getInsertPt());
widenScalarDst(MI, WideTy, 1);
Observer.changedInstr(MI);
return Legalized;
After widening, we must insert 2 G_TRUNC operations. First G_TRUNC is inserted at correct position. widenScalarDst() increments next insertion position. Therefore, the second widenScalarDst() inserts 2nd G_TRUNC 1 instruction too late. This leads to following incorrect code:
%2:_(s16) = G_TRUNC %10:_(s32)
%4:_(s16) = G_INTRINSIC intrinsic(@llvm.custom.fma), %2:_(s16), %3:_(s16), %2:_(s16)
%3:_(s16) = G_TRUNC %11:_(s32)
My proposed fix is in the comment above. This applies to G_UDIVREM as well.
To submit the patch, I need a test. But I'm unable to reproduce this on backends other than my custom backend. So what's the right way to submit this test?