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07aca076e2
Summary: This implements an optimization described in Hacker's Delight 10-17: when `C` is constant, the result of `X % C == 0` can be computed more cheaply without actually calculating the remainder. The motivation is discussed here: https://bugs.llvm.org/show_bug.cgi?id=35479. One huge caveat: this signed case is only valid for positive divisors. While we can freely negate negative divisors, we can't negate `INT_MIN`, so for now if `INT_MIN` is encountered, we bailout. As a follow-up, it should be possible to handle that more gracefully via extra `and`+`setcc`+`select`. This passes llvm's test-suite, and from cursory(!) cross-examination the folds (the assembly) match those of GCC, and manual checking via alive did not reveal any issues (other than the `INT_MIN` case) Reviewers: RKSimon, spatel, hermord, craig.topper, xbolva00 Reviewed By: RKSimon, xbolva00 Subscribers: xbolva00, thakis, javed.absar, hiraditya, dexonsmith, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D65366 llvm-svn: 368702
74 lines
2.2 KiB
LLVM
74 lines
2.2 KiB
LLVM
; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
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; RUN: llc -mtriple=i686-unknown-linux-gnu < %s | FileCheck %s --check-prefixes=CHECK,X86
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; RUN: llc -mtriple=x86_64-unknown-linux-gnu < %s | FileCheck %s --check-prefixes=CHECK,X64
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; On X86, division in expensive. BuildRemEqFold should therefore run even
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; when optimizing for size. Only optimizing for minimum size retains a plain div.
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define i32 @test_minsize(i32 %X) optsize minsize nounwind readnone {
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; X86-LABEL: test_minsize:
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; X86: # %bb.0:
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; X86-NEXT: movl {{[0-9]+}}(%esp), %eax
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; X86-NEXT: pushl $5
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; X86-NEXT: popl %ecx
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; X86-NEXT: cltd
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; X86-NEXT: idivl %ecx
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; X86-NEXT: testl %edx, %edx
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; X86-NEXT: je .LBB0_1
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; X86-NEXT: # %bb.2:
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; X86-NEXT: pushl $-10
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; X86-NEXT: popl %eax
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; X86-NEXT: retl
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; X86-NEXT: .LBB0_1:
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; X86-NEXT: pushl $42
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; X86-NEXT: popl %eax
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; X86-NEXT: retl
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;
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; X64-LABEL: test_minsize:
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; X64: # %bb.0:
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; X64-NEXT: movl %edi, %eax
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; X64-NEXT: pushq $5
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; X64-NEXT: popq %rcx
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; X64-NEXT: cltd
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; X64-NEXT: idivl %ecx
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; X64-NEXT: testl %edx, %edx
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; X64-NEXT: pushq $42
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; X64-NEXT: popq %rcx
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; X64-NEXT: pushq $-10
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; X64-NEXT: popq %rax
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; X64-NEXT: cmovel %ecx, %eax
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; X64-NEXT: retq
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%rem = srem i32 %X, 5
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%cmp = icmp eq i32 %rem, 0
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%ret = select i1 %cmp, i32 42, i32 -10
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ret i32 %ret
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}
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define i32 @test_optsize(i32 %X) optsize nounwind readnone {
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; X86-LABEL: test_optsize:
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; X86: # %bb.0:
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; X86-NEXT: imull $-858993459, {{[0-9]+}}(%esp), %eax # imm = 0xCCCCCCCD
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; X86-NEXT: addl $429496729, %eax # imm = 0x19999999
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; X86-NEXT: cmpl $858993459, %eax # imm = 0x33333333
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; X86-NEXT: movl $42, %eax
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; X86-NEXT: jb .LBB1_2
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; X86-NEXT: # %bb.1:
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; X86-NEXT: movl $-10, %eax
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; X86-NEXT: .LBB1_2:
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; X86-NEXT: retl
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;
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; X64-LABEL: test_optsize:
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; X64: # %bb.0:
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; X64-NEXT: imull $-858993459, %edi, %eax # imm = 0xCCCCCCCD
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; X64-NEXT: addl $429496729, %eax # imm = 0x19999999
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; X64-NEXT: cmpl $858993459, %eax # imm = 0x33333333
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; X64-NEXT: movl $42, %ecx
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; X64-NEXT: movl $-10, %eax
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; X64-NEXT: cmovbl %ecx, %eax
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; X64-NEXT: retq
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%rem = srem i32 %X, 5
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%cmp = icmp eq i32 %rem, 0
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%ret = select i1 %cmp, i32 42, i32 -10
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ret i32 %ret
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}
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