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[CGP] despeculateCountZeros - Don't create is-zero branch if cttz/ctlz source is known non-zero

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If value tracking can confirm that the cttz/ctlz source is known non-zero then we don't need to create a branch (which DAG will struggle to recover from).

Differential Revision: https://reviews.llvm.org/D106685
This commit is contained in:
Simon Pilgrim 2021-07-24 12:58:02 +01:00
parent bf642a4547
commit 5740094940
2 changed files with 4 additions and 22 deletions
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4
lib/CodeGen/CodeGenPrepare.cpp
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@ -2041,6 +2041,10 @@ static bool despeculateCountZeros(IntrinsicInst *CountZeros,
if (Ty->isVectorTy() || SizeInBits > DL->getLargestLegalIntTypeSizeInBits())
return false;
// Bail if the value is never zero.
if (llvm::isKnownNonZero(CountZeros->getOperand(0), *DL))
return false;
// The intrinsic will be sunk behind a compare against zero and branch.
BasicBlock *StartBlock = CountZeros->getParent();
BasicBlock *CallBlock = StartBlock->splitBasicBlock(CountZeros, "cond.false");

22
test/CodeGen/X86/clz.ll
View File

@ -704,33 +704,21 @@ define i64 @cttz_i64_zero_test(i64 %n) {
; Don't generate the cmovne when the source is known non-zero (and bsr would
; not set ZF).
; rdar://9490949
; FIXME: The compare and branch are produced late in IR (by CodeGenPrepare), and
; codegen doesn't know how to delete the movl and je.
define i32 @ctlz_i32_fold_cmov(i32 %n) {
; X86-LABEL: ctlz_i32_fold_cmov:
; X86: # %bb.0:
; X86-NEXT: movl {{[0-9]+}}(%esp), %eax
; X86-NEXT: orl $1, %eax
; X86-NEXT: je .LBB16_1
; X86-NEXT: # %bb.2: # %cond.false
; X86-NEXT: bsrl %eax, %eax
; X86-NEXT: xorl $31, %eax
; X86-NEXT: retl
; X86-NEXT: .LBB16_1:
; X86-NEXT: movl $32, %eax
; X86-NEXT: retl
;
; X64-LABEL: ctlz_i32_fold_cmov:
; X64: # %bb.0:
; X64-NEXT: orl $1, %edi
; X64-NEXT: je .LBB16_1
; X64-NEXT: # %bb.2: # %cond.false
; X64-NEXT: bsrl %edi, %eax
; X64-NEXT: xorl $31, %eax
; X64-NEXT: retq
; X64-NEXT: .LBB16_1:
; X64-NEXT: movl $32, %eax
; X64-NEXT: retq
;
; X86-CLZ-LABEL: ctlz_i32_fold_cmov:
; X86-CLZ: # %bb.0:
@ -953,14 +941,9 @@ define i64 @ctlz_i64_zero_test_knownneverzero(i64 %n) {
; X64-LABEL: ctlz_i64_zero_test_knownneverzero:
; X64: # %bb.0:
; X64-NEXT: orq $1, %rdi
; X64-NEXT: je .LBB21_1
; X64-NEXT: # %bb.2: # %cond.false
; X64-NEXT: bsrq %rdi, %rax
; X64-NEXT: xorq $63, %rax
; X64-NEXT: retq
; X64-NEXT: .LBB21_1:
; X64-NEXT: movl $64, %eax
; X64-NEXT: retq
;
; X86-CLZ-LABEL: ctlz_i64_zero_test_knownneverzero:
; X86-CLZ: # %bb.0:
@ -1026,13 +1009,8 @@ define i64 @cttz_i64_zero_test_knownneverzero(i64 %n) {
; X64: # %bb.0:
; X64-NEXT: movabsq $-9223372036854775808, %rax # imm = 0x8000000000000000
; X64-NEXT: orq %rdi, %rax
; X64-NEXT: je .LBB22_1
; X64-NEXT: # %bb.2: # %cond.false
; X64-NEXT: bsfq %rax, %rax
; X64-NEXT: retq
; X64-NEXT: .LBB22_1:
; X64-NEXT: movl $64, %eax
; X64-NEXT: retq
;
; X86-CLZ-LABEL: cttz_i64_zero_test_knownneverzero:
; X86-CLZ: # %bb.0: