From 56442dfdcf7b07c04b585de5205b9427b1739895 Mon Sep 17 00:00:00 2001 From: Benjamin Kramer Date: Wed, 2 Feb 2011 15:56:22 +0000 Subject: [PATCH] SimplifyCFG: Turn switches into sub+icmp+branch if possible. This makes the job of the later optzn passes easier, allowing the vast amount of icmp transforms to chew on it. We transform 840 switches in gcc.c, leading to a 16k byte shrink of the resulting binary on i386-linux. The testcase from README.txt now compiles into decl %edi cmpl $3, %edi sbbl %eax, %eax andl $1, %eax ret git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@124724 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Target/README.txt | 48 ------------------- lib/Transforms/Utils/SimplifyCFG.cpp | 32 +++++++++++++ test/Transforms/SimplifyCFG/switch_create.ll | 17 ++----- .../SimplifyCFG/switch_formation.dbg.ll | 12 ++--- 4 files changed, 39 insertions(+), 70 deletions(-) diff --git a/lib/Target/README.txt b/lib/Target/README.txt index b3bc7498566..c0a2b760de7 100644 --- a/lib/Target/README.txt +++ b/lib/Target/README.txt @@ -2274,51 +2274,3 @@ llc time when it gets inlined, because we can use smaller transfers. This also avoids partial register stalls in some important cases. //===---------------------------------------------------------------------===// - -With PR8575 we're now generating better code for: - -static _Bool foo(int x) { return x == 1; } -static _Bool bar(int x) { return x == 2; } -static _Bool baz(int x) { return x == 3; } - -_Bool quux(int x) { - return foo(x) || bar(x) || baz(x); -} - -$ clang t.c -S -o - -O3 -mkernel -fomit-frame-pointer -_quux: ## @quux -## BB#0: ## %entry - decl %edi - cmpl $3, %edi - movb $1, %al - jb LBB0_2 -## BB#1: ## %lor.rhs - xorb %al, %al -LBB0_2: ## %lor.end - movzbl %al, %eax - andl $1, %eax - ret - -But this should use a "setcc" instead of materializing a 0/1 value -the hard way. This looks like #1: simplifycfg should transform the -switch into a sub+icmp+branch, and an instcombine hack to replace -the PHI with a zext of the branch condition. Here's the IR today: - -define zeroext i1 @quux(i32 %x) nounwind readnone ssp noredzone { -entry: - switch i32 %x, label %lor.rhs [ - i32 1, label %lor.end - i32 2, label %lor.end - i32 3, label %lor.end - ] - -lor.rhs: ; preds = %entry - br label %lor.end - -lor.end: ; preds = %lor.rhs, %entry, %entry, %entry - %0 = phi i1 [ true, %entry ], [ false, %lor.rhs ], [ true, %entry ], [ true, %entry ] - ret i1 %0 -} - -//===---------------------------------------------------------------------===// - diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp index b9432c2e6e1..bf753dc05d4 100644 --- a/lib/Transforms/Utils/SimplifyCFG.cpp +++ b/lib/Transforms/Utils/SimplifyCFG.cpp @@ -2237,6 +2237,34 @@ bool SimplifyCFGOpt::SimplifyUnreachable(UnreachableInst *UI) { return Changed; } +/// TurnSwitchRangeIntoICmp - Turns a switch with that contains only a +/// integer range comparison into a sub, an icmp and a branch. +static bool TurnSwitchRangeIntoICmp(SwitchInst *SI) { + assert(SI->getNumCases() > 2 && "Degenerate switch?"); + // We can do this transform if the switch consists of an ascending series + // and all cases point to the same destination. + for (unsigned I = 2, E = SI->getNumCases(); I != E; ++I) + if (SI->getSuccessor(I-1) != SI->getSuccessor(I) || + SI->getCaseValue(I-1)->getValue()+1 != SI->getCaseValue(I)->getValue()) + return false; + + Constant *Offset = ConstantExpr::getNeg(SI->getCaseValue(1)); + Constant *NumCases = ConstantInt::get(Offset->getType(), SI->getNumCases()-1); + + Value *Sub = BinaryOperator::CreateAdd(SI->getCondition(), Offset, "off", SI); + Value *Cmp = new ICmpInst(SI, ICmpInst::ICMP_ULT, Sub, NumCases, "switch"); + BranchInst::Create(SI->getSuccessor(1), SI->getDefaultDest(), Cmp, SI); + + // Prune obsolete incoming values off the successor's PHI nodes. + for (BasicBlock::iterator BBI = SI->getSuccessor(1)->begin(); + isa(BBI); ++BBI) { + for (unsigned I = 0, E = SI->getNumCases()-2; I != E; ++I) + cast(BBI)->removeIncomingValue(SI->getParent()); + } + SI->eraseFromParent(); + + return true; +} bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI) { // If this switch is too complex to want to look at, ignore it. @@ -2260,6 +2288,10 @@ bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI) { if (SI == &*BBI) if (FoldValueComparisonIntoPredecessors(SI)) return SimplifyCFG(BB) | true; + + // Try to transform the switch into an icmp and a branch. + if (TurnSwitchRangeIntoICmp(SI)) + return SimplifyCFG(BB) | true; return false; } diff --git a/test/Transforms/SimplifyCFG/switch_create.ll b/test/Transforms/SimplifyCFG/switch_create.ll index 4e199bc8596..7c153e86822 100644 --- a/test/Transforms/SimplifyCFG/switch_create.ll +++ b/test/Transforms/SimplifyCFG/switch_create.ll @@ -141,14 +141,8 @@ UnifiedReturnBlock: ; preds = %shortcirc_done.4, %shortcirc_next.4 ret i1 %UnifiedRetVal ; CHECK: @test6 -; CHECK: switch i32 %tmp.2.i, label %shortcirc_next.4 [ -; CHECK: i32 14, label %UnifiedReturnBlock -; CHECK: i32 15, label %UnifiedReturnBlock -; CHECK: i32 16, label %UnifiedReturnBlock -; CHECK: i32 17, label %UnifiedReturnBlock -; CHECK: i32 18, label %UnifiedReturnBlock -; CHECK: i32 19, label %UnifiedReturnBlock -; CHECK: ] +; CHECK: %off = add i32 %tmp.2.i, -14 +; CHECK: %switch = icmp ult i32 %off, 6 } define void @test7(i8 zeroext %c, i32 %x) nounwind ssp noredzone { @@ -447,11 +441,8 @@ if.end: define zeroext i1 @test16(i32 %x) nounwind { entry: ; CHECK: @test16 -; CHECK: switch i32 %x, label %lor.rhs [ -; CHECK: i32 1, label %lor.end -; CHECK: i32 2, label %lor.end -; CHECK: i32 3, label %lor.end -; CHECK: ] +; CHECK: %off = add i32 %x, -1 +; CHECK: %switch = icmp ult i32 %off, 3 %cmp.i = icmp eq i32 %x, 1 br i1 %cmp.i, label %lor.end, label %lor.lhs.false diff --git a/test/Transforms/SimplifyCFG/switch_formation.dbg.ll b/test/Transforms/SimplifyCFG/switch_formation.dbg.ll index 357ffb60e1e..09bef648ab0 100644 --- a/test/Transforms/SimplifyCFG/switch_formation.dbg.ll +++ b/test/Transforms/SimplifyCFG/switch_formation.dbg.ll @@ -13,15 +13,9 @@ declare void @llvm.dbg.stoppoint(i32, i32, { }*) nounwind define i1 @t({ i32, i32 }* %I) { -; CHECK: t -; CHECK: switch i32 %tmp.2.i, label %shortcirc_next.4 [ -; CHECK: i32 14, label %UnifiedReturnBlock -; CHECK: i32 15, label %UnifiedReturnBlock -; CHECK: i32 16, label %UnifiedReturnBlock -; CHECK: i32 17, label %UnifiedReturnBlock -; CHECK: i32 18, label %UnifiedReturnBlock -; CHECK: i32 19, label %UnifiedReturnBlock -; CHECK: ] +; CHECK: @t +; CHECK: %off = add i32 %tmp.2.i, -14 +; CHECK: %switch = icmp ult i32 %off, 6 entry: %tmp.1.i = getelementptr { i32, i32 }* %I, i64 0, i32 1 ; [#uses=1] %tmp.2.i = load i32* %tmp.1.i ; [#uses=6]