[llvm] [SPIR-V] Add pre-headers to loops. (PR #75844)
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Mon Dec 18 11:16:28 PST 2023
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-backend-spir-v
Author: Nathan Gauër (Keenuts)
<details>
<summary>Changes</summary>
This is the first of the 7 steps outlined in #<!-- -->75801. This PR explicitely calls the SimplifyLoops pass. Directly following this pass should follow the 6 others required to structurize the IR.
Running this pass could generate empty basic-blocks, which are implicit fallthrough to the successor BB.
There was a specific condition in the SPIR-V ISel which handled implicit fallthrough, but it couldn't work on empty basic-blocks. This commits removes the old logic, and adds this new logic, which checks all basic-blocks for implicit fallthroughs, including empty ones.
---
Full diff: https://github.com/llvm/llvm-project/pull/75844.diff
5 Files Affected:
- (modified) llvm/lib/Target/SPIRV/SPIRVPreLegalizer.cpp (+35)
- (modified) llvm/lib/Target/SPIRV/SPIRVTargetMachine.cpp (+6)
- (modified) llvm/lib/Target/SPIRV/SPIRVUtils.cpp (+2-2)
- (modified) llvm/lib/Target/SPIRV/SPIRVUtils.h (+1-1)
- (added) llvm/test/CodeGen/SPIRV/scfg-add-pre-headers.ll (+67)
``````````diff
diff --git a/llvm/lib/Target/SPIRV/SPIRVPreLegalizer.cpp b/llvm/lib/Target/SPIRV/SPIRVPreLegalizer.cpp
index f4076be2a7b778..275c24e22c3025 100644
--- a/llvm/lib/Target/SPIRV/SPIRVPreLegalizer.cpp
+++ b/llvm/lib/Target/SPIRV/SPIRVPreLegalizer.cpp
@@ -587,6 +587,40 @@ static void processSwitches(MachineFunction &MF, SPIRVGlobalRegistry *GR,
}
}
+static bool isImplicitFallthrough(MachineBasicBlock &MBB) {
+ if (MBB.empty())
+ return true;
+
+ // Branching spirv intrinsics are not detected by this generic method.
+ // Thus, we can only trust negative result.
+ if (!MBB.canFallThrough())
+ return false;
+
+ // Otherwise, we must manually check if we have a spirv intrinsic which
+ // prevent an implicit fallthrough.
+ for (MachineBasicBlock::reverse_iterator It = MBB.rbegin(), E = MBB.rend();
+ It != E; ++It) {
+ if (isSpvIntrinsic(*It, Intrinsic::spv_switch))
+ return false;
+ }
+ return true;
+}
+
+static void removeImplicitFallthroughs(MachineFunction &MF,
+ MachineIRBuilder MIB) {
+ // It is valid for MachineBasicBlocks to not finish with a branch instruction.
+ // In such cases, they will simply fallthrough their immediate successor.
+ for (MachineBasicBlock &MBB : MF) {
+ if (!isImplicitFallthrough(MBB))
+ continue;
+
+ assert(std::distance(MBB.successors().begin(), MBB.successors().end()) ==
+ 1);
+ MIB.setInsertPt(MBB, MBB.end());
+ MIB.buildBr(**MBB.successors().begin());
+ }
+}
+
bool SPIRVPreLegalizer::runOnMachineFunction(MachineFunction &MF) {
// Initialize the type registry.
const SPIRVSubtarget &ST = MF.getSubtarget<SPIRVSubtarget>();
@@ -599,6 +633,7 @@ bool SPIRVPreLegalizer::runOnMachineFunction(MachineFunction &MF) {
generateAssignInstrs(MF, GR, MIB);
processSwitches(MF, GR, MIB);
processInstrsWithTypeFolding(MF, GR, MIB);
+ removeImplicitFallthroughs(MF, MIB);
return true;
}
diff --git a/llvm/lib/Target/SPIRV/SPIRVTargetMachine.cpp b/llvm/lib/Target/SPIRV/SPIRVTargetMachine.cpp
index 1503f263e42c0d..517174b356f059 100644
--- a/llvm/lib/Target/SPIRV/SPIRVTargetMachine.cpp
+++ b/llvm/lib/Target/SPIRV/SPIRVTargetMachine.cpp
@@ -29,6 +29,7 @@
#include "llvm/MC/TargetRegistry.h"
#include "llvm/Pass.h"
#include "llvm/Target/TargetOptions.h"
+#include "llvm/Transforms/Utils.h"
#include <optional>
using namespace llvm;
@@ -151,6 +152,11 @@ TargetPassConfig *SPIRVTargetMachine::createPassConfig(PassManagerBase &PM) {
}
void SPIRVPassConfig::addIRPasses() {
+ // Once legalized, we need to structurize the CFG to follow the spec.
+ // This is done through the following 8 steps.
+ // TODO(#75801): add the remaining steps.
+ addPass(createLoopSimplifyPass());
+
TargetPassConfig::addIRPasses();
addPass(createSPIRVRegularizerPass());
addPass(createSPIRVPrepareFunctionsPass(TM));
diff --git a/llvm/lib/Target/SPIRV/SPIRVUtils.cpp b/llvm/lib/Target/SPIRV/SPIRVUtils.cpp
index 1c0e8d84e2fd10..d4f7d8e89af5e4 100644
--- a/llvm/lib/Target/SPIRV/SPIRVUtils.cpp
+++ b/llvm/lib/Target/SPIRV/SPIRVUtils.cpp
@@ -228,8 +228,8 @@ uint64_t getIConstVal(Register ConstReg, const MachineRegisterInfo *MRI) {
return MI->getOperand(1).getCImm()->getValue().getZExtValue();
}
-bool isSpvIntrinsic(MachineInstr &MI, Intrinsic::ID IntrinsicID) {
- if (auto *GI = dyn_cast<GIntrinsic>(&MI))
+bool isSpvIntrinsic(const MachineInstr &MI, Intrinsic::ID IntrinsicID) {
+ if (const auto *GI = dyn_cast<GIntrinsic>(&MI))
return GI->is(IntrinsicID);
return false;
}
diff --git a/llvm/lib/Target/SPIRV/SPIRVUtils.h b/llvm/lib/Target/SPIRV/SPIRVUtils.h
index 30fae6c7de479f..60742e2f272808 100644
--- a/llvm/lib/Target/SPIRV/SPIRVUtils.h
+++ b/llvm/lib/Target/SPIRV/SPIRVUtils.h
@@ -79,7 +79,7 @@ MachineInstr *getDefInstrMaybeConstant(Register &ConstReg,
uint64_t getIConstVal(Register ConstReg, const MachineRegisterInfo *MRI);
// Check if MI is a SPIR-V specific intrinsic call.
-bool isSpvIntrinsic(MachineInstr &MI, Intrinsic::ID IntrinsicID);
+bool isSpvIntrinsic(const MachineInstr &MI, Intrinsic::ID IntrinsicID);
// Get type of i-th operand of the metadata node.
Type *getMDOperandAsType(const MDNode *N, unsigned I);
diff --git a/llvm/test/CodeGen/SPIRV/scfg-add-pre-headers.ll b/llvm/test/CodeGen/SPIRV/scfg-add-pre-headers.ll
new file mode 100644
index 00000000000000..eccf3301002ae7
--- /dev/null
+++ b/llvm/test/CodeGen/SPIRV/scfg-add-pre-headers.ll
@@ -0,0 +1,67 @@
+; RUN: llc -mtriple=spirv-unknown-unknown -O0 %s -o - | FileCheck %s
+
+; CHECK-DAG: [[bool:%[0-9]+]] = OpTypeBool
+; CHECK-DAG: [[uint:%[0-9]+]] = OpTypeInt 32 0
+; CHECK-DAG: [[uint_0:%[0-9]+]] = OpConstant [[uint]] 0
+
+define i32 @main(i32 noundef %0) #1 {
+ %2 = icmp ne i32 %0, 0
+ br i1 %2, label %l1, label %l2
+
+; CHECK: [[param_0:%[0-9]+]] = OpFunctionParameter [[uint]]
+; CHECK: [[cond:%[0-9]+]] = OpINotEqual [[bool]] [[param_0]] [[uint_0]]
+; CHECK: OpBranchConditional [[cond]] [[l1_pre:%[0-9]+]] [[l2_pre:%[0-9]+]]
+
+; CHECK-DAG: [[l2_pre]] = OpLabel
+; CHECK-NEXT: OpBranch [[l2_header:%[0-9]+]]
+
+; CHECK-DAG: [[l1_pre]] = OpLabel
+; CHECK-NEXT: OpBranch [[l1_header:%[0-9]+]]
+
+l1:
+ br i1 %2, label %l1_body, label %l1_end
+; CHECK-DAG: [[l1_header]] = OpLabel
+; CHECK-NEXT: OpBranchConditional [[cond]] [[l1_body:%[0-9]+]] [[l1_end:%[0-9]+]]
+
+l1_body:
+ br label %l1_continue
+; CHECK-DAG: [[l1_body]] = OpLabel
+; CHECK-NEXT: OpBranch [[l1_continue:%[0-9]+]]
+
+l1_continue:
+ br label %l1
+; CHECK-DAG: [[l1_continue]] = OpLabel
+; CHECK-NEXT: OpBranch [[l1_header]]
+
+l1_end:
+ br label %end
+; CHECK-DAG: [[l1_end]] = OpLabel
+; CHECK-NEXT: OpBranch [[end:%[0-9]+]]
+
+l2:
+ br i1 %2, label %l2_body, label %l2_end
+; CHECK-DAG: [[l2_header]] = OpLabel
+; CHECK-NEXT: OpBranchConditional [[cond]] [[l2_body:%[0-9]+]] [[l2_end:%[0-9]+]]
+
+l2_body:
+ br label %l2_continue
+; CHECK-DAG: [[l2_body]] = OpLabel
+; CHECK-NEXT: OpBranch [[l2_continue:%[0-9]+]]
+
+l2_continue:
+ br label %l2
+; CHECK-DAG: [[l2_continue]] = OpLabel
+; CHECK-NEXT: OpBranch [[l2_header]]
+
+l2_end:
+ br label %end
+; CHECK-DAG: [[l2_end]] = OpLabel
+; CHECK-NEXT: OpBranch [[end:%[0-9]+]]
+
+end:
+ ret i32 1
+; CHECK-DAG: [[end]] = OpLabel
+; CHECK-NEXT: OpReturn
+}
+
+attributes #1 = { "hlsl.numthreads"="4,8,16" "hlsl.shader"="compute" convergent }
``````````
</details>
https://github.com/llvm/llvm-project/pull/75844
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