From 12d79f7de4a722b84473500751434d4794d1f702 Mon Sep 17 00:00:00 2001 From: Alex Crichton Date: Sun, 14 Oct 2018 12:27:22 -0700 Subject: [PATCH] rustc: Fix (again) simd vectors by-val in ABI The issue of passing around SIMD types as values between functions has seen [quite a lot] of [discussion], and although we thought [we fixed it][quite a lot] it [wasn't]! This PR is a change to rustc to, again, try to fix this issue. The fundamental problem here remains the same, if a SIMD vector argument is passed by-value in LLVM's function type, then if the caller and callee disagree on target features a miscompile happens. We solve this by never passing SIMD vectors by-value, but LLVM will still thwart us with its argument promotion pass to promote by-ref SIMD arguments to by-val SIMD arguments. This commit is an attempt to thwart LLVM thwarting us. We, just before codegen, will take yet another look at the LLVM module and demote any by-value SIMD arguments we see. This is a very manual attempt by us to ensure the codegen for a module keeps working, and it unfortunately is likely producing suboptimal code, even in release mode. The saving grace for this, in theory, is that if SIMD types are passed by-value across a boundary in release mode it's pretty unlikely to be performance sensitive (as it's already doing a load/store, and otherwise perf-sensitive bits should be inlined). The implementation here is basically a big wad of C++. It was largely copied from LLVM's own argument promotion pass, only doing the reverse. In local testing this... Closes #50154 Closes #52636 Closes #54583 Closes #55059 [quite a lot]: https://github.com/rust-lang/rust/pull/47743 [discussion]: https://github.com/rust-lang/rust/issues/44367 [wasn't]: https://github.com/rust-lang/rust/issues/50154 --- src/librustc_codegen_llvm/back/lto.rs | 12 +- src/librustc_codegen_llvm/back/write.rs | 34 +++- src/librustc_codegen_llvm/llvm/ffi.rs | 2 + src/librustc_llvm/build.rs | 4 +- src/rustllvm/DemoteSimd.cpp | 181 ++++++++++++++++++ .../simd-argument-promotion-thwarted/Makefile | 13 ++ .../simd-argument-promotion-thwarted/t1.rs | 21 ++ .../simd-argument-promotion-thwarted/t2.rs | 14 ++ .../simd-argument-promotion-thwarted/t3.rs | 52 +++++ 9 files changed, 324 insertions(+), 9 deletions(-) create mode 100644 src/rustllvm/DemoteSimd.cpp create mode 100644 src/test/run-make/simd-argument-promotion-thwarted/Makefile create mode 100644 src/test/run-make/simd-argument-promotion-thwarted/t1.rs create mode 100644 src/test/run-make/simd-argument-promotion-thwarted/t2.rs create mode 100644 src/test/run-make/simd-argument-promotion-thwarted/t3.rs diff --git a/src/librustc_codegen_llvm/back/lto.rs b/src/librustc_codegen_llvm/back/lto.rs index 3ac22f4eaef65..f4afe52135876 100644 --- a/src/librustc_codegen_llvm/back/lto.rs +++ b/src/librustc_codegen_llvm/back/lto.rs @@ -80,9 +80,7 @@ impl LtoModuleCodegen { let module = module.take().unwrap(); { let config = cgcx.config(module.kind); - let llmod = module.module_llvm.llmod(); - let tm = &*module.module_llvm.tm; - run_pass_manager(cgcx, tm, llmod, config, false); + run_pass_manager(cgcx, &module, config, false); timeline.record("fat-done"); } Ok(module) @@ -557,8 +555,7 @@ fn thin_lto(cgcx: &CodegenContext, } fn run_pass_manager(cgcx: &CodegenContext, - tm: &llvm::TargetMachine, - llmod: &llvm::Module, + module: &ModuleCodegen, config: &ModuleConfig, thin: bool) { // Now we have one massive module inside of llmod. Time to run the @@ -569,7 +566,8 @@ fn run_pass_manager(cgcx: &CodegenContext, debug!("running the pass manager"); unsafe { let pm = llvm::LLVMCreatePassManager(); - llvm::LLVMRustAddAnalysisPasses(tm, pm, llmod); + let llmod = module.module_llvm.llmod(); + llvm::LLVMRustAddAnalysisPasses(module.module_llvm.tm, pm, llmod); if config.verify_llvm_ir { let pass = llvm::LLVMRustFindAndCreatePass("verify\0".as_ptr() as *const _); @@ -864,7 +862,7 @@ impl ThinModule { // little differently. info!("running thin lto passes over {}", module.name); let config = cgcx.config(module.kind); - run_pass_manager(cgcx, module.module_llvm.tm, llmod, config, true); + run_pass_manager(cgcx, &module, config, true); cgcx.save_temp_bitcode(&module, "thin-lto-after-pm"); timeline.record("thin-done"); } diff --git a/src/librustc_codegen_llvm/back/write.rs b/src/librustc_codegen_llvm/back/write.rs index 02ef690b94233..f4a7afc1667a3 100644 --- a/src/librustc_codegen_llvm/back/write.rs +++ b/src/librustc_codegen_llvm/back/write.rs @@ -633,7 +633,7 @@ unsafe fn optimize(cgcx: &CodegenContext, None, &format!("llvm module passes [{}]", module_name.unwrap()), || { - llvm::LLVMRunPassManager(mpm, llmod) + llvm::LLVMRunPassManager(mpm, llmod); }); // Deallocate managers that we're now done with @@ -691,6 +691,38 @@ unsafe fn codegen(cgcx: &CodegenContext, create_msvc_imps(cgcx, llcx, llmod); } + // Ok now this one's a super interesting invocations. SIMD in rustc is + // difficult where we want some parts of the program to be able to use + // some SIMD features while other parts of the program don't. The real + // tough part is that we want this to actually work correctly! + // + // We go to great lengths to make sure this works, and one crucial + // aspect is that vector arguments (simd types) are never passed by + // value in the ABI of functions. It turns out, however, that LLVM will + // undo our "clever work" of passing vector types by reference. Its + // argument promotion pass will promote these by-ref arguments to + // by-val. That, however, introduces codegen errors! + // + // The upstream LLVM bug [1] has unfortunatey not really seen a lot of + // activity. The Rust bug [2], however, has seen quite a lot of reports + // of this in the wild. As a result, this is worked around locally here. + // We have a custom transformation, `LLVMRustDemoteSimdArguments`, which + // does the opposite of argument promotion by demoting any by-value SIMD + // arguments in function signatures to pointers intead of being + // by-value. + // + // This operates at the LLVM IR layer because LLVM is thwarting our + // codegen and this is the only chance we get to make sure it's correct + // before we hit codegen. + // + // Hopefully one day the upstream LLVM bug will be fixed and we'll no + // longer need this! + // + // [1]: https://bugs.llvm.org/show_bug.cgi?id=37358 + // [2]: https://github.com/rust-lang/rust/issues/50154 + llvm::LLVMRustDemoteSimdArguments(llmod); + cgcx.save_temp_bitcode(&module, "simd-demoted"); + // A codegen-specific pass manager is used to generate object // files for an LLVM module. // diff --git a/src/librustc_codegen_llvm/llvm/ffi.rs b/src/librustc_codegen_llvm/llvm/ffi.rs index c9f51efdc5095..3286377ad326f 100644 --- a/src/librustc_codegen_llvm/llvm/ffi.rs +++ b/src/librustc_codegen_llvm/llvm/ffi.rs @@ -1136,6 +1136,8 @@ extern "C" { /// Runs a pass manager on a module. pub fn LLVMRunPassManager(PM: &PassManager<'a>, M: &'a Module) -> Bool; + pub fn LLVMRustDemoteSimdArguments(M: &'a Module); + pub fn LLVMInitializePasses(); pub fn LLVMPassManagerBuilderCreate() -> &'static mut PassManagerBuilder; diff --git a/src/librustc_llvm/build.rs b/src/librustc_llvm/build.rs index 7d01ed556c8dd..ad5db19839ef0 100644 --- a/src/librustc_llvm/build.rs +++ b/src/librustc_llvm/build.rs @@ -162,7 +162,9 @@ fn main() { } build_helper::rerun_if_changed_anything_in_dir(Path::new("../rustllvm")); - cfg.file("../rustllvm/PassWrapper.cpp") + cfg + .file("../rustllvm/DemoteSimd.cpp") + .file("../rustllvm/PassWrapper.cpp") .file("../rustllvm/RustWrapper.cpp") .file("../rustllvm/ArchiveWrapper.cpp") .file("../rustllvm/Linker.cpp") diff --git a/src/rustllvm/DemoteSimd.cpp b/src/rustllvm/DemoteSimd.cpp new file mode 100644 index 0000000000000..3c75376077932 --- /dev/null +++ b/src/rustllvm/DemoteSimd.cpp @@ -0,0 +1,181 @@ +// Copyright 2018 The Rust Project Developers. See the COPYRIGHT +// file at the top-level directory of this distribution and at +// http://rust-lang.org/COPYRIGHT. +// +// Licensed under the Apache License, Version 2.0 or the MIT license +// , at your +// option. This file may not be copied, modified, or distributed +// except according to those terms. + +#include +#include + +#include "rustllvm.h" + +#include "llvm/IR/CallSite.h" +#include "llvm/IR/Module.h" +#include "llvm/ADT/STLExtras.h" + +using namespace llvm; + +static std::vector +GetFunctionsWithSimdArgs(Module *M) { + std::vector Ret; + + for (auto &F : M->functions()) { + // Skip all intrinsic calls as these are always tightly controlled to "work + // correctly", so no need to fixup any of these. + if (F.isIntrinsic()) + continue; + + // We're only interested in rustc-defined functions, not unstably-defined + // imported SIMD ffi functions. + if (F.isDeclaration()) + continue; + + // Argument promotion only happens on internal functions, so skip demoting + // arguments in external functions like FFI shims and such. + if (!F.hasLocalLinkage()) + continue; + + // If any argument to this function is a by-value vector type, then that's + // bad! The compiler didn't generate any functions that looked like this, + // and we try to rely on LLVM to not do this! Argument promotion may, + // however, promote arguments from behind references. In any case, figure + // out if we're interested in demoting this argument. + if (any_of(F.args(), [](Argument &arg) { return arg.getType()->isVectorTy(); })) + Ret.push_back(&F); + } + + return Ret; +} + +extern "C" void +LLVMRustDemoteSimdArguments(LLVMModuleRef Mod) { + Module *M = unwrap(Mod); + + auto Functions = GetFunctionsWithSimdArgs(M); + + for (auto F : Functions) { + // Build up our list of new parameters and new argument attributes. + // We're only changing those arguments which are vector types. + SmallVector Params; + SmallVector ArgAttrVec; + auto PAL = F->getAttributes(); + for (auto &Arg : F->args()) { + auto *Ty = Arg.getType(); + if (Ty->isVectorTy()) { + Params.push_back(PointerType::get(Ty, 0)); + ArgAttrVec.push_back(AttributeSet()); + } else { + Params.push_back(Ty); + ArgAttrVec.push_back(PAL.getParamAttributes(Arg.getArgNo())); + } + } + + // Replace `F` with a new function with our new signature. I'm... not really + // sure how this works, but this is all the steps `ArgumentPromotion` does + // to replace a signature as well. + assert(!F->isVarArg()); // ArgumentPromotion should skip these fns + FunctionType *NFTy = FunctionType::get(F->getReturnType(), Params, false); + Function *NF = Function::Create(NFTy, F->getLinkage(), F->getName()); + NF->copyAttributesFrom(F); + NF->setSubprogram(F->getSubprogram()); + F->setSubprogram(nullptr); + NF->setAttributes(AttributeList::get(F->getContext(), + PAL.getFnAttributes(), + PAL.getRetAttributes(), + ArgAttrVec)); + ArgAttrVec.clear(); + F->getParent()->getFunctionList().insert(F->getIterator(), NF); + NF->takeName(F); + + // Iterate over all invocations of `F`, updating all `call` instructions to + // store immediate vector types in a local `alloc` instead of a by-value + // vector. + // + // Like before, much of this is copied from the `ArgumentPromotion` pass in + // LLVM. + SmallVector Args; + while (!F->use_empty()) { + CallSite CS(F->user_back()); + assert(CS.getCalledFunction() == F); + Instruction *Call = CS.getInstruction(); + const AttributeList &CallPAL = CS.getAttributes(); + + // Loop over the operands, inserting an `alloca` and a store for any + // argument we're demoting to be by reference + // + // FIXME: we probably want to figure out an LLVM pass to run and clean up + // this function and instructions we're generating, we should in theory + // only generate a maximum number of `alloca` instructions rather than + // one-per-variable unconditionally. + CallSite::arg_iterator AI = CS.arg_begin(); + size_t ArgNo = 0; + for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; + ++I, ++AI, ++ArgNo) { + if (I->getType()->isVectorTy()) { + AllocaInst *AllocA = new AllocaInst(I->getType(), 0, nullptr, "", Call); + new StoreInst(*AI, AllocA, Call); + Args.push_back(AllocA); + ArgAttrVec.push_back(AttributeSet()); + } else { + Args.push_back(*AI); + ArgAttrVec.push_back(CallPAL.getParamAttributes(ArgNo)); + } + } + assert(AI == CS.arg_end()); + + // Create a new call instructions which we'll use to replace the old call + // instruction, copying over as many attributes and such as possible. + SmallVector OpBundles; + CS.getOperandBundlesAsDefs(OpBundles); + + CallSite NewCS; + if (InvokeInst *II = dyn_cast(Call)) { + InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(), + Args, OpBundles, "", Call); + } else { + auto *NewCall = CallInst::Create(NF, Args, OpBundles, "", Call); + NewCall->setTailCallKind(cast(Call)->getTailCallKind()); + NewCS = NewCall; + } + NewCS.setCallingConv(CS.getCallingConv()); + NewCS.setAttributes( + AttributeList::get(F->getContext(), CallPAL.getFnAttributes(), + CallPAL.getRetAttributes(), ArgAttrVec)); + NewCS->setDebugLoc(Call->getDebugLoc()); + Args.clear(); + ArgAttrVec.clear(); + Call->replaceAllUsesWith(NewCS.getInstruction()); + NewCS->takeName(Call); + Call->eraseFromParent(); + } + + // Splice the body of the old function right into the new function. + NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList()); + + // Update our new function to replace all uses of the by-value argument with + // loads of the pointer argument we've generated. + // + // FIXME: we probably want to only generate one load instruction per + // function? Or maybe run an LLVM pass to clean up this function? + for (Function::arg_iterator I = F->arg_begin(), + E = F->arg_end(), + I2 = NF->arg_begin(); + I != E; + ++I, ++I2) { + if (I->getType()->isVectorTy()) { + I->replaceAllUsesWith(new LoadInst(&*I2, "", &NF->begin()->front())); + } else { + I->replaceAllUsesWith(&*I2); + } + I2->takeName(&*I); + } + + // Delete all references to the old function, it should be entirely dead + // now. + M->getFunctionList().remove(F); + } +} diff --git a/src/test/run-make/simd-argument-promotion-thwarted/Makefile b/src/test/run-make/simd-argument-promotion-thwarted/Makefile new file mode 100644 index 0000000000000..3095432d0fe69 --- /dev/null +++ b/src/test/run-make/simd-argument-promotion-thwarted/Makefile @@ -0,0 +1,13 @@ +-include ../../run-make-fulldeps/tools.mk + +ifeq ($(TARGET),x86_64-unknown-linux-gnu) +all: + $(RUSTC) t1.rs -C opt-level=3 + $(TMPDIR)/t1 + $(RUSTC) t2.rs -C opt-level=3 + $(TMPDIR)/t2 + $(RUSTC) t3.rs -C opt-level=3 + $(TMPDIR)/t3 +else +all: +endif diff --git a/src/test/run-make/simd-argument-promotion-thwarted/t1.rs b/src/test/run-make/simd-argument-promotion-thwarted/t1.rs new file mode 100644 index 0000000000000..cb4a3dd7d4a7c --- /dev/null +++ b/src/test/run-make/simd-argument-promotion-thwarted/t1.rs @@ -0,0 +1,21 @@ +use std::arch::x86_64; + +fn main() { + if !is_x86_feature_detected!("avx2") { + return println!("AVX2 is not supported on this machine/build."); + } + let load_bytes: [u8; 32] = [0x0f; 32]; + let lb_ptr = load_bytes.as_ptr(); + let reg_load = unsafe { + x86_64::_mm256_loadu_si256( + lb_ptr as *const x86_64::__m256i + ) + }; + println!("{:?}", reg_load); + let mut store_bytes: [u8; 32] = [0; 32]; + let sb_ptr = store_bytes.as_mut_ptr(); + unsafe { + x86_64::_mm256_storeu_si256(sb_ptr as *mut x86_64::__m256i, reg_load); + } + assert_eq!(load_bytes, store_bytes); +} diff --git a/src/test/run-make/simd-argument-promotion-thwarted/t2.rs b/src/test/run-make/simd-argument-promotion-thwarted/t2.rs new file mode 100644 index 0000000000000..0e42b82a223d0 --- /dev/null +++ b/src/test/run-make/simd-argument-promotion-thwarted/t2.rs @@ -0,0 +1,14 @@ +use std::arch::x86_64::*; + +fn main() { + if !is_x86_feature_detected!("avx") { + return println!("AVX is not supported on this machine/build."); + } + unsafe { + let f = _mm256_set_pd(2.0, 2.0, 2.0, 2.0); + let r = _mm256_mul_pd(f, f); + + union A { a: __m256d, b: [f64; 4] } + assert_eq!(A { a: r }.b, [4.0, 4.0, 4.0, 4.0]); + } +} diff --git a/src/test/run-make/simd-argument-promotion-thwarted/t3.rs b/src/test/run-make/simd-argument-promotion-thwarted/t3.rs new file mode 100644 index 0000000000000..10062ab3e4643 --- /dev/null +++ b/src/test/run-make/simd-argument-promotion-thwarted/t3.rs @@ -0,0 +1,52 @@ +use std::arch::x86_64::*; + +#[target_feature(enable = "avx")] +unsafe fn avx_mul(a: __m256, b: __m256) -> __m256 { + _mm256_mul_ps(a, b) +} + +#[target_feature(enable = "avx")] +unsafe fn avx_store(p: *mut f32, a: __m256) { + _mm256_storeu_ps(p, a) +} + +#[target_feature(enable = "avx")] +unsafe fn avx_setr(a: f32, b: f32, c: f32, d: f32, e: f32, f: f32, g: f32, h: f32) -> __m256 { + _mm256_setr_ps(a, b, c, d, e, f, g, h) +} + +#[target_feature(enable = "avx")] +unsafe fn avx_set1(a: f32) -> __m256 { + _mm256_set1_ps(a) +} + +struct Avx(__m256); + +fn mul(a: Avx, b: Avx) -> Avx { + unsafe { Avx(avx_mul(a.0, b.0)) } +} + +fn set1(a: f32) -> Avx { + unsafe { Avx(avx_set1(a)) } +} + +fn setr(a: f32, b: f32, c: f32, d: f32, e: f32, f: f32, g: f32, h: f32) -> Avx { + unsafe { Avx(avx_setr(a, b, c, d, e, f, g, h)) } +} + +unsafe fn store(p: *mut f32, a: Avx) { + avx_store(p, a.0); +} + +fn main() { + if !is_x86_feature_detected!("avx") { + return println!("AVX is not supported on this machine/build."); + } + let mut result = [0.0f32; 8]; + let a = mul(setr(0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0), set1(0.25)); + unsafe { + store(result.as_mut_ptr(), a); + } + + assert_eq!(result, [0.0, 0.25, 0.5, 0.75, 1.0, 1.25, 1.50, 1.75]); +}