[Matrix] Check if iterator is at beginning of BB in optimizeTranspose.

[fhahn]

Summary

If an instruction at the beginning of a block is erased, this may
trigger crash due to dereferencing an invalid iterator.

Check if II is at the end before dereferencing it.

[thegameg]

LGTM thanks!

llvm/lib/Transforms/Scalar/LowerMatrixIntrinsics.cpp

void optimizeTransposes() {
     // First sink all transposes inside matmuls, hoping that we end up with NN,
     // NT or TN variants.
     for (BasicBlock &BB : reverse(Func)) {
       for (auto II = BB.rbegin(); II != BB.rend();) {
         Instruction &I = *II;
         // We may remove II.  By default continue on the next/prev instruction.
         ++II;
         // If we were to erase II, move again.
-        auto EraseFromParent = [&II](Value *V) {
+        auto EraseFromParent = [&II, &BB](Value *V) {
           auto *Inst = cast<Instruction>(V);
           if (Inst->use_empty()) {
-            if (Inst == &*II) {
+            if (II != BB.rend() && Inst == &*II) {
               ++II;
             }
             Inst->eraseFromParent();
           }
         };
 
         // If we're creating a new instruction, continue from there.
         Instruction *NewInst = nullptr;

llvm/test/Transforms/LowerMatrixIntrinsics/transpose-opts-iterator-invalidation.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -passes=lower-matrix-intrinsics -S %s | FileCheck %s
+
+define <2 x double> @test(<4 x double> %a, <2 x double> %b, i1 %c) {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[C:%.*]], label [[EXIT_1:%.*]], label [[EXIT_2:%.*]]
+; CHECK:       exit.1:
+; CHECK-NEXT:    [[SPLIT:%.*]] = shufflevector <4 x double> [[A:%.*]], <4 x double> poison, <2 x i32> <i32 0, i32 1>
+; CHECK-NEXT:    [[SPLIT1:%.*]] = shufflevector <4 x double> [[A]], <4 x double> poison, <2 x i32> <i32 2, i32 3>
+; CHECK-NEXT:    [[SPLIT2:%.*]] = shufflevector <2 x double> [[B:%.*]], <2 x double> poison, <2 x i32> <i32 0, i32 1>
+; CHECK-NEXT:    [[BLOCK:%.*]] = shufflevector <2 x double> [[SPLIT]], <2 x double> poison, <1 x i32> zeroinitializer
+; CHECK-NEXT:    [[TMP0:%.*]] = extractelement <2 x double> [[SPLIT2]], i64 0
+; CHECK-NEXT:    [[SPLAT_SPLATINSERT:%.*]] = insertelement <1 x double> poison, double [[TMP0]], i32 0
+; CHECK-NEXT:    [[SPLAT_SPLAT:%.*]] = shufflevector <1 x double> [[SPLAT_SPLATINSERT]], <1 x double> poison, <1 x i32> zeroinitializer
+; CHECK-NEXT:    [[TMP1:%.*]] = fmul <1 x double> [[BLOCK]], [[SPLAT_SPLAT]]
+; CHECK-NEXT:    [[BLOCK3:%.*]] = shufflevector <2 x double> [[SPLIT1]], <2 x double> poison, <1 x i32> zeroinitializer
+; CHECK-NEXT:    [[TMP2:%.*]] = extractelement <2 x double> [[SPLIT2]], i64 1
+; CHECK-NEXT:    [[SPLAT_SPLATINSERT4:%.*]] = insertelement <1 x double> poison, double [[TMP2]], i32 0
+; CHECK-NEXT:    [[SPLAT_SPLAT5:%.*]] = shufflevector <1 x double> [[SPLAT_SPLATINSERT4]], <1 x double> poison, <1 x i32> zeroinitializer
+; CHECK-NEXT:    [[TMP3:%.*]] = fmul <1 x double> [[BLOCK3]], [[SPLAT_SPLAT5]]
+; CHECK-NEXT:    [[TMP4:%.*]] = fadd <1 x double> [[TMP1]], [[TMP3]]
+; CHECK-NEXT:    [[TMP5:%.*]] = shufflevector <1 x double> [[TMP4]], <1 x double> poison, <2 x i32> <i32 0, i32 undef>
+; CHECK-NEXT:    [[TMP6:%.*]] = shufflevector <2 x double> undef, <2 x double> [[TMP5]], <2 x i32> <i32 2, i32 1>
+; CHECK-NEXT:    [[BLOCK6:%.*]] = shufflevector <2 x double> [[SPLIT]], <2 x double> poison, <1 x i32> <i32 1>
+; CHECK-NEXT:    [[TMP7:%.*]] = extractelement <2 x double> [[SPLIT2]], i64 0
+; CHECK-NEXT:    [[SPLAT_SPLATINSERT7:%.*]] = insertelement <1 x double> poison, double [[TMP7]], i32 0
+; CHECK-NEXT:    [[SPLAT_SPLAT8:%.*]] = shufflevector <1 x double> [[SPLAT_SPLATINSERT7]], <1 x double> poison, <1 x i32> zeroinitializer
+; CHECK-NEXT:    [[TMP8:%.*]] = fmul <1 x double> [[BLOCK6]], [[SPLAT_SPLAT8]]
+; CHECK-NEXT:    [[BLOCK9:%.*]] = shufflevector <2 x double> [[SPLIT1]], <2 x double> poison, <1 x i32> <i32 1>
+; CHECK-NEXT:    [[TMP9:%.*]] = extractelement <2 x double> [[SPLIT2]], i64 1
+; CHECK-NEXT:    [[SPLAT_SPLATINSERT10:%.*]] = insertelement <1 x double> poison, double [[TMP9]], i32 0
+; CHECK-NEXT:    [[SPLAT_SPLAT11:%.*]] = shufflevector <1 x double> [[SPLAT_SPLATINSERT10]], <1 x double> poison, <1 x i32> zeroinitializer
+; CHECK-NEXT:    [[TMP10:%.*]] = fmul <1 x double> [[BLOCK9]], [[SPLAT_SPLAT11]]
+; CHECK-NEXT:    [[TMP11:%.*]] = fadd <1 x double> [[TMP8]], [[TMP10]]
+; CHECK-NEXT:    [[TMP12:%.*]] = shufflevector <1 x double> [[TMP11]], <1 x double> poison, <2 x i32> <i32 0, i32 undef>
+; CHECK-NEXT:    [[TMP13:%.*]] = shufflevector <2 x double> [[TMP6]], <2 x double> [[TMP12]], <2 x i32> <i32 0, i32 2>
+; CHECK-NEXT:    [[TMP14:%.*]] = extractelement <2 x double> [[TMP13]], i64 0
+; CHECK-NEXT:    [[TMP15:%.*]] = insertelement <1 x double> undef, double [[TMP14]], i64 0
+; CHECK-NEXT:    [[TMP16:%.*]] = extractelement <2 x double> [[TMP13]], i64 1
+; CHECK-NEXT:    [[TMP17:%.*]] = insertelement <1 x double> undef, double [[TMP16]], i64 0
+; CHECK-NEXT:    [[TMP18:%.*]] = shufflevector <1 x double> [[TMP15]], <1 x double> [[TMP17]], <2 x i32> <i32 0, i32 1>
+; CHECK-NEXT:    ret <2 x double> [[TMP18]]
+; CHECK:       exit.2:
+; CHECK-NEXT:    ret <2 x double> zeroinitializer
+;
+entry:
+  %mult = call <2 x double> @llvm.matrix.multiply.v2f64.v4f64.v2f64(<4 x double> %a, <2 x double> %b, i32 2, i32 2, i32 1)
+  br i1 %c, label %exit.1, label %exit.2
+
+exit.1:
+  %trans = call <2 x double> @llvm.matrix.transpose.v2f64(<2 x double> %mult, i32 2, i32 1)
+  ret <2 x double> %trans
+
+exit.2:
+  ret <2 x double> zeroinitializer
+}
+
+declare <2 x double> @llvm.matrix.transpose.v2f64(<2 x double>, i32 immarg, i32 immarg)
+
+declare <2 x double> @llvm.matrix.multiply.v2f64.v4f64.v2f64(<4 x double>, <2 x double>, i32 immarg, i32 immarg, i32 immarg)