[GISel][RISCV] Fix several boundary cases in narrow G_SEXT_INREG. (llvm#72719)

This fixes cases when SizeInBits is a multiple of the narrow size.

If SizeBits is equal to NarrowTy size, the first block would create an
illegal G_SEXT_INREG where the the extension size is equal to the type.
I tried to turn it into G_TRUNC+G_SEXT, but that just turned back into
G_SEXT_INREG causing an infinite loop. So punt to the splitting case.

In the for loop we should copy when the part ends on SizeInBits. In that
case there is no G_SEXT_INREG needed for partial. But we should note
that register in PartialExtensionReg for the first full part to use.

If the part starts on SizeInBits then we should do an AShr of
PartialExtensionReg.

We should only get to the G_SEXT_INREG case if the SizeInBits is in the
middle of the part.

Author: topperc

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

@@ -1445,7 +1445,7 @@ LegalizerHelper::LegalizeResult LegalizerHelper::narrowScalar(MachineInstr &MI,
 
     // So long as the new type has more bits than the bits we're extending we
     // don't need to break it apart.
-    if (NarrowTy.getScalarSizeInBits() >= SizeInBits) {
+    if (NarrowTy.getScalarSizeInBits() > SizeInBits) {
       Observer.changingInstr(MI);
       // We don't lose any non-extension bits by truncating the src and
       // sign-extending the dst.
@@ -1488,14 +1488,15 @@ LegalizerHelper::LegalizeResult LegalizerHelper::narrowScalar(MachineInstr &MI,
     Register AshrCstReg =
         MIRBuilder.buildConstant(NarrowTy, NarrowTy.getScalarSizeInBits() - 1)
             .getReg(0);
-    Register FullExtensionReg = 0;
-    Register PartialExtensionReg = 0;
+    Register FullExtensionReg;
+    Register PartialExtensionReg;
 
     // Do the operation on each small part.
     for (int i = 0; i < NumParts; ++i) {
-      if ((i + 1) * NarrowTy.getScalarSizeInBits() < SizeInBits)
+      if ((i + 1) * NarrowTy.getScalarSizeInBits() <= SizeInBits) {
         DstRegs.push_back(SrcRegs[i]);
-      else if (i * NarrowTy.getScalarSizeInBits() > SizeInBits) {
+        PartialExtensionReg = DstRegs.back();
+      } else if (i * NarrowTy.getScalarSizeInBits() >= SizeInBits) {
         assert(PartialExtensionReg &&
                "Expected to visit partial extension before full");
         if (FullExtensionReg) {

llvm/test/CodeGen/RISCV/GlobalISel/legalizer/legalize-mulo-rv32.mir

@@ -168,16 +168,13 @@ body:             |
     ; LIBCALL-NEXT: ADJCALLSTACKUP 0, 0, implicit-def $x2, implicit $x2
     ; LIBCALL-NEXT: [[COPY2:%[0-9]+]]:_(s32) = COPY $x10
     ; LIBCALL-NEXT: [[COPY3:%[0-9]+]]:_(s32) = COPY $x11
-    ; LIBCALL-NEXT: [[C2:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
-    ; LIBCALL-NEXT: [[SHL:%[0-9]+]]:_(s32) = G_SHL [[COPY2]], [[C2]](s32)
-    ; LIBCALL-NEXT: [[ASHR2:%[0-9]+]]:_(s32) = G_ASHR [[SHL]], [[C2]](s32)
-    ; LIBCALL-NEXT: [[C3:%[0-9]+]]:_(s32) = G_CONSTANT i32 31
-    ; LIBCALL-NEXT: [[ASHR3:%[0-9]+]]:_(s32) = G_ASHR [[ASHR2]], [[C3]](s32)
-    ; LIBCALL-NEXT: [[C4:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
-    ; LIBCALL-NEXT: [[XOR:%[0-9]+]]:_(s32) = G_XOR [[COPY2]], [[ASHR2]]
-    ; LIBCALL-NEXT: [[XOR1:%[0-9]+]]:_(s32) = G_XOR [[COPY3]], [[ASHR3]]
+    ; LIBCALL-NEXT: [[C2:%[0-9]+]]:_(s32) = G_CONSTANT i32 31
+    ; LIBCALL-NEXT: [[ASHR2:%[0-9]+]]:_(s32) = G_ASHR [[COPY2]], [[C2]](s32)
+    ; LIBCALL-NEXT: [[C3:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
+    ; LIBCALL-NEXT: [[XOR:%[0-9]+]]:_(s32) = G_XOR [[COPY2]], [[COPY2]]
+    ; LIBCALL-NEXT: [[XOR1:%[0-9]+]]:_(s32) = G_XOR [[COPY3]], [[ASHR2]]
     ; LIBCALL-NEXT: [[OR:%[0-9]+]]:_(s32) = G_OR [[XOR]], [[XOR1]]
-    ; LIBCALL-NEXT: [[ICMP:%[0-9]+]]:_(s32) = G_ICMP intpred(ne), [[OR]](s32), [[C4]]
+    ; LIBCALL-NEXT: [[ICMP:%[0-9]+]]:_(s32) = G_ICMP intpred(ne), [[OR]](s32), [[C3]]
     ; LIBCALL-NEXT: $x10 = COPY [[COPY2]](s32)
     ; LIBCALL-NEXT: $x11 = COPY [[ICMP]](s32)
     ; LIBCALL-NEXT: PseudoRET implicit $x10, implicit $x11

llvm/test/CodeGen/RISCV/GlobalISel/legalizer/legalize-mulo-rv64.mir

@@ -218,16 +218,13 @@ body:             |
     ; LIBCALL-NEXT: ADJCALLSTACKUP 0, 0, implicit-def $x2, implicit $x2
     ; LIBCALL-NEXT: [[COPY2:%[0-9]+]]:_(s64) = COPY $x10
     ; LIBCALL-NEXT: [[COPY3:%[0-9]+]]:_(s64) = COPY $x11
-    ; LIBCALL-NEXT: [[C2:%[0-9]+]]:_(s64) = G_CONSTANT i64 0
-    ; LIBCALL-NEXT: [[SHL:%[0-9]+]]:_(s64) = G_SHL [[COPY2]], [[C2]](s64)
-    ; LIBCALL-NEXT: [[ASHR2:%[0-9]+]]:_(s64) = G_ASHR [[SHL]], [[C2]](s64)
-    ; LIBCALL-NEXT: [[C3:%[0-9]+]]:_(s64) = G_CONSTANT i64 63
-    ; LIBCALL-NEXT: [[ASHR3:%[0-9]+]]:_(s64) = G_ASHR [[ASHR2]], [[C3]](s64)
-    ; LIBCALL-NEXT: [[C4:%[0-9]+]]:_(s64) = G_CONSTANT i64 0
-    ; LIBCALL-NEXT: [[XOR:%[0-9]+]]:_(s64) = G_XOR [[COPY2]], [[ASHR2]]
-    ; LIBCALL-NEXT: [[XOR1:%[0-9]+]]:_(s64) = G_XOR [[COPY3]], [[ASHR3]]
+    ; LIBCALL-NEXT: [[C2:%[0-9]+]]:_(s64) = G_CONSTANT i64 63
+    ; LIBCALL-NEXT: [[ASHR2:%[0-9]+]]:_(s64) = G_ASHR [[COPY2]], [[C2]](s64)
+    ; LIBCALL-NEXT: [[C3:%[0-9]+]]:_(s64) = G_CONSTANT i64 0
+    ; LIBCALL-NEXT: [[XOR:%[0-9]+]]:_(s64) = G_XOR [[COPY2]], [[COPY2]]
+    ; LIBCALL-NEXT: [[XOR1:%[0-9]+]]:_(s64) = G_XOR [[COPY3]], [[ASHR2]]
     ; LIBCALL-NEXT: [[OR:%[0-9]+]]:_(s64) = G_OR [[XOR]], [[XOR1]]
-    ; LIBCALL-NEXT: [[ICMP:%[0-9]+]]:_(s64) = G_ICMP intpred(ne), [[OR]](s64), [[C4]]
+    ; LIBCALL-NEXT: [[ICMP:%[0-9]+]]:_(s64) = G_ICMP intpred(ne), [[OR]](s64), [[C3]]
     ; LIBCALL-NEXT: $x10 = COPY [[COPY2]](s64)
     ; LIBCALL-NEXT: $x11 = COPY [[ICMP]](s64)
     ; LIBCALL-NEXT: PseudoRET implicit $x10, implicit $x11