b3c54914ef
This is a small code size optimization that lets us avoid both shifting and comparing to a constant if we need the shifted value anyway. On most architectures the zero comparison is cheaper than a constant comparison (or free if the shift sets flags). Although this change appears to remove the optimization entirely, we continue to do this transform if there is one use because of the code below the removed code that transforms the shift into an and, followed by the PR10267 case in InstCombinerImpl::foldICmpAndConstConst that transforms the and into a ult/ugt. Added a test case to verify this explicitly. Per [1] reduces clang .text size by 0.09% and dynamic instruction count by 0.01%. [1] https://llvm-compile-time-tracker.com/compare.php?from=1f38d49ebe96417e368a567efa4d650b8a9ac30f&to=0873787a12b8f2eab019d8211ace4bccc1807343&stat=size-text Reviewers: nikic, dtcxzyw Reviewed By: dtcxzyw Pull Request: https://github.com/llvm/llvm-project/pull/168007
70 lines
2.6 KiB
LLVM
70 lines
2.6 KiB
LLVM
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
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; RUN: opt < %s -O1 -S -mattr=+lzcnt | FileCheck %s
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target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
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target triple = "x86_64-unknown-linux-gnu"
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; This test ensures we are able to optimize the following loop to an llvm.abs
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; followed by an llvm.ctlz.
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; FIXME: LoopIdiom recongize is not forming llvm.ctlz.
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; int ctlz_zero_check(int n)
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; {
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; n = n >= 0 ? n : -n;
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; int i = 0;
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; while(n) {
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; n >>= 1;
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; i++;
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; }
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; return i;
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; }
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define i32 @ctlz_loop_with_abs(i32 %n) {
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; CHECK-LABEL: @ctlz_loop_with_abs(
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; CHECK-NEXT: entry:
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; CHECK-NEXT: [[TOBOOL_NOT1:%.*]] = icmp eq i32 [[N:%.*]], 0
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; CHECK-NEXT: br i1 [[TOBOOL_NOT1]], label [[WHILE_END:%.*]], label [[WHILE_BODY_PREHEADER:%.*]]
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; CHECK: while.body.preheader:
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; CHECK-NEXT: [[TMP0:%.*]] = tail call i32 @llvm.abs.i32(i32 [[N]], i1 true)
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; CHECK-NEXT: br label [[WHILE_BODY:%.*]]
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; CHECK: while.body:
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; CHECK-NEXT: [[N_ADDR_03:%.*]] = phi i32 [ [[TMP1:%.*]], [[WHILE_BODY]] ], [ [[TMP0]], [[WHILE_BODY_PREHEADER]] ]
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; CHECK-NEXT: [[I_02:%.*]] = phi i32 [ [[INC:%.*]], [[WHILE_BODY]] ], [ 0, [[WHILE_BODY_PREHEADER]] ]
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; CHECK-NEXT: [[TMP1]] = lshr i32 [[N_ADDR_03]], 1
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; CHECK-NEXT: [[INC]] = add nuw nsw i32 [[I_02]], 1
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; CHECK-NEXT: [[TOBOOL_NOT:%.*]] = icmp eq i32 [[TMP1]], 0
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; CHECK-NEXT: br i1 [[TOBOOL_NOT]], label [[WHILE_END]], label [[WHILE_BODY]]
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; CHECK: while.end:
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; CHECK-NEXT: [[I_0_LCSSA:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[INC]], [[WHILE_BODY]] ]
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; CHECK-NEXT: ret i32 [[I_0_LCSSA]]
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;
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entry:
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%cmp = icmp sge i32 %n, 0
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br i1 %cmp, label %cond.true, label %cond.false
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cond.true: ; preds = %entry
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br label %cond.end
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cond.false: ; preds = %entry
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%sub = sub nsw i32 0, %n
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br label %cond.end
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cond.end: ; preds = %cond.false, %cond.true
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%cond = phi i32 [ %n, %cond.true ], [ %sub, %cond.false ]
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br label %while.cond
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while.cond: ; preds = %while.body, %cond.end
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%i.0 = phi i32 [ 0, %cond.end ], [ %inc, %while.body ]
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%n.addr.0 = phi i32 [ %cond, %cond.end ], [ %shr, %while.body ]
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%tobool = icmp ne i32 %n.addr.0, 0
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br i1 %tobool, label %while.body, label %while.end
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while.body: ; preds = %while.cond
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%shr = ashr i32 %n.addr.0, 1
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%inc = add nsw i32 %i.0, 1
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br label %while.cond
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while.end: ; preds = %while.cond
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ret i32 %i.0
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}
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