[Passes] Fix relative lookup table converter pass

This patch fixes the relative table converter pass for the lookup table
accesses that are resulted in an instruction sequence, where gep is not
immediately followed by a load, such as gep being hoisted outside the loop
or another instruction is inserted in between them. The fix inserts the
call to load.relative.instrinsic in the original place of load instead of gep.
Issue is reported by FreeBSD via https://bugs.freebsd.org/259921.

Differential Revision: https://reviews.llvm.org/D115571

(cherry picked from commit e5a8af7)

Author: gulfemsavrun

llvm/lib/Transforms/Utils/RelLookupTableConverter.cpp

@@ -144,6 +144,10 @@ static void convertToRelLookupTable(GlobalVariable &LookupTable) {
   Value *Offset =
       Builder.CreateShl(Index, ConstantInt::get(IntTy, 2), "reltable.shift");
 
+  // Insert the call to load.relative instrinsic before LOAD.
+  // GEP might not be immediately followed by a LOAD, like it can be hoisted
+  // outside the loop or another instruction might be inserted them in between.
+  Builder.SetInsertPoint(Load);
   Function *LoadRelIntrinsic = llvm::Intrinsic::getDeclaration(
       &M, Intrinsic::load_relative, {Index->getType()});
   Value *Base = Builder.CreateBitCast(RelLookupTable, Builder.getInt8PtrTy());

llvm/test/Transforms/RelLookupTableConverter/X86/relative_lookup_table.ll

@@ -12,6 +12,9 @@ target triple = "x86_64-unknown-linux-gnu"
 @.str.5 = private unnamed_addr constant [5 x i8] c"str1\00", align 1
 @.str.6 = private unnamed_addr constant [5 x i8] c"str2\00", align 1
 @.str.7 = private unnamed_addr constant [12 x i8] c"singlevalue\00", align 1
+@.str.8 = private unnamed_addr constant [2 x i8] c"a\00", align 1
+@.str.9 = private unnamed_addr constant [2 x i8] c"b\00", align 1
+@.str.10 = private unnamed_addr constant [2 x i8] c"c\00", align 1
 
 @a1 = external global i32, align 4
 @b1 = external global i32, align 4
@@ -56,6 +59,16 @@ target triple = "x86_64-unknown-linux-gnu"
                                     i8* getelementptr inbounds ([4 x i8], [4 x i8]* @.str.2, i32 0, i32 0)
                                    ], align 16
 
+@table = internal constant [2 x i8*] [
+  i8* getelementptr inbounds ([2 x i8], [2 x i8]* @.str.8, i32 0, i32 0),
+  i8* getelementptr inbounds ([2 x i8], [2 x i8]* @.str.9, i32 0, i32 0)
+], align 16
+
+@table2 = internal constant [2 x i8*] [
+  i8* getelementptr inbounds ([2 x i8], [2 x i8]* @.str.8, i32 0, i32 0),
+  i8* getelementptr inbounds ([2 x i8], [2 x i8]* @.str.9, i32 0, i32 0)
+], align 16
+
 ; Lookup table check for integer pointers that have external linkage
 ; CHECK: @switch.table.external_linkage = private unnamed_addr constant [3 x i32*] [i32* @a1, i32* @b1, i32* @c1], align
 
@@ -93,6 +106,20 @@ target triple = "x86_64-unknown-linux-gnu"
 ; CHECK-SAME: ], align 4
 ;
 
+; Relative lookup table for the loop hoist check test
+; CHECK: @reltable.loop_hoist = internal unnamed_addr constant [2 x i32]
+; CHECK-SAME: [
+; CHECK-SAME: i32 trunc (i64 sub (i64 ptrtoint ([2 x i8]* @.str.8 to i64), i64 ptrtoint ([2 x i32]* @reltable.loop_hoist to i64)) to i32),
+; CHECK-SAME: i32 trunc (i64 sub (i64 ptrtoint ([2 x i8]* @.str.9 to i64), i64 ptrtoint ([2 x i32]* @reltable.loop_hoist to i64)) to i32)
+; CHECK-SAME: ], align 4
+
+; Relative look up table for the test where gep is not immediately followed by a load check
+; CHECK: @reltable.gep_is_not_imm_followed_by_load = internal unnamed_addr constant [2 x i32]
+; CHECK-SAME: [
+; CHECK-SAME: i32 trunc (i64 sub (i64 ptrtoint ([2 x i8]* @.str.8 to i64), i64 ptrtoint ([2 x i32]* @reltable.gep_is_not_imm_followed_by_load to i64)) to i32),
+; CHECK-SAME: i32 trunc (i64 sub (i64 ptrtoint ([2 x i8]* @.str.9 to i64), i64 ptrtoint ([2 x i32]* @reltable.gep_is_not_imm_followed_by_load to i64)) to i32)
+; CHECK-SAME: ], align 4
+
 ; Lookup table check for integer pointers that have external linkage
 define i32* @external_linkage(i32 %cond) {
 ; CHECK-LABEL: @external_linkage(
@@ -260,6 +287,59 @@ cond.end:                                         ; preds = %entry, %cond.false
   ret i8* %cond1
 }
 
+; Check to ensure that call @llvm.load.relative is inserted before load, not before gep.
+; When a lookup table is accessed inside a loop, and a gep is hosted outside the loop via licm,
+; make sure that call @llvm.load.relative is inserted before load.
+define i8* @loop_hoist(i32 %x) {
+; CHECK-LABEL: @loop_hoist(i32 %x)
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp sgt i32 [[X:%.*]], 1
+; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds [2 x i8], [2 x i8]* @.str.10, i32 0, i32 0
+; CHECK-NEXT:    [[RELTABLE_SHIFT:%.*]] = shl i32 [[X:%.*]], 2
+; CHECK-NEXT:    br i1 [[TMP0]], label %if.done, label %if.false
+; CHECK:       if.false:
+; CHECK-NEXT:    [[RELTABLE_INTRINSIC:%.*]] = call i8* @llvm.load.relative.i32(i8* bitcast ([2 x i32]* @reltable.loop_hoist to i8*), i32 [[RELTABLE_SHIFT]])
+; CHECK-NEXT:    br label %if.done
+; CHECK:       if.done:
+; CHECK-NEXT:    [[TMP2:%.*]] = phi i8* [ [[TMP1]], %entry ], [ [[RELTABLE_INTRINSIC]], %if.false ]
+; CHECK-NEXT:    ret i8* [[TMP2]]
+;
+entry:
+  %0 = icmp sgt i32 %x, 1
+  %1 = getelementptr inbounds [2 x i8], [2 x i8]* @.str.10, i32 0, i32 0
+  %2 = getelementptr [2 x i8*], [2 x i8*]* @table, i32 0, i32 %x
+  br i1 %0, label %if.done, label %if.false
+
+if.false:
+  %3 = load i8*, i8** %2
+  br label %if.done
+
+if.done:
+  %4 = phi i8* [ %1, %entry ], [ %3, %if.false ]
+  ret i8* %4
+}
+
+; Another check to ensure that call @llvm.load.relative is inserted before load but not before gep.
+; When a lookup table is accessed, and gep is not immediately followed by a load (like if there is a function call
+; or an exception in between), make sure that call @llvm.load.relative is inserted before load.
+; CHECK-LABEL: @may_not_return()
+declare void @may_not_return()
+
+define i8* @gep_is_not_imm_followed_by_load(i32 %x) {
+; CHECK-LABEL: @gep_is_not_imm_followed_by_load(i32 %x)
+; CHECK:       entry:
+; CHECK-NEXT:    [[RELTABLE_SHIFT:%.*]] = shl i32 [[X:%.*]], 2
+; CHECK-NEXT:    call void @may_not_return()
+; CHECK-NEXT:    [[RELTABLE_INTRINSIC:%.*]] = call i8* @llvm.load.relative.i32(i8* bitcast ([2 x i32]* @reltable.gep_is_not_imm_followed_by_load to i8*), i32 [[RELTABLE_SHIFT]])
+; CHECK-NEXT:    ret i8* [[RELTABLE_INTRINSIC]]
+;
+entry:
+  %0 = getelementptr [2 x i8*], [2 x i8*]* @table2, i32 0, i32 %x
+  call void @may_not_return()
+  %1 = load i8*, i8** %0
+  ret i8* %1
+}
+
 !llvm.module.flags = !{!0, !1}
 !0 = !{i32 7, !"PIC Level", i32 2}
 !1 = !{i32 1, !"Code Model", i32 1}