[llvm] Do not emit right shift by 0 in DWARF expressions (PR #162738)
Tom Tromey via llvm-commits
llvm-commits at lists.llvm.org
Thu Oct 9 14:20:18 PDT 2025
https://github.com/tromey created https://github.com/llvm/llvm-project/pull/162738
DW_OP_LLVM_extract_bits_zext and DW_OP_LLVM_extract_bits_sext can end up emitting "DW_OP_constu 0; DW_OP_shr" when given certain arguments. However, a shift by zero is not useful, and so it can be omitted.
>From eb3f34766b6596fdbdc06ab0d5f6fdc36785a419 Mon Sep 17 00:00:00 2001
From: Tom Tromey <tromey at adacore.com>
Date: Thu, 9 Oct 2025 14:04:48 -0600
Subject: [PATCH] Do not emit right shift by 0 in DWARF expressions
DW_OP_LLVM_extract_bits_zext and DW_OP_LLVM_extract_bits_sext can end
up emitting "DW_OP_constu 0; DW_OP_shr" when given certain arguments.
However, a shift by zero is not useful, and so it can be omitted.
---
llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp | 10 ++++++----
llvm/test/DebugInfo/X86/DW_OP_LLVM_extract_bits.ll | 5 +++++
2 files changed, 11 insertions(+), 4 deletions(-)
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
index bc0bb349be249..f0f086184656a 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
@@ -587,10 +587,12 @@ bool DwarfExpression::addExpression(
emitUnsigned(LeftShift);
emitOp(dwarf::DW_OP_shl);
}
- emitOp(dwarf::DW_OP_constu);
- emitUnsigned(RightShift);
- emitOp(OpNum == dwarf::DW_OP_LLVM_extract_bits_sext ? dwarf::DW_OP_shra
- : dwarf::DW_OP_shr);
+ if (RightShift) {
+ emitOp(dwarf::DW_OP_constu);
+ emitUnsigned(RightShift);
+ emitOp(OpNum == dwarf::DW_OP_LLVM_extract_bits_sext ? dwarf::DW_OP_shra
+ : dwarf::DW_OP_shr);
+ }
// The value is now at the top of the stack, so set the location to
// implicit so that we get a stack_value at the end.
diff --git a/llvm/test/DebugInfo/X86/DW_OP_LLVM_extract_bits.ll b/llvm/test/DebugInfo/X86/DW_OP_LLVM_extract_bits.ll
index 8342d42f6a292..5928127866f7e 100644
--- a/llvm/test/DebugInfo/X86/DW_OP_LLVM_extract_bits.ll
+++ b/llvm/test/DebugInfo/X86/DW_OP_LLVM_extract_bits.ll
@@ -67,6 +67,8 @@ entry:
; CHECK: DW_TAG_variable
; CHECK: DW_AT_location (DW_OP_fbreg -4, DW_OP_plus_uconst 0x3, DW_OP_deref_size 0x1, DW_OP_constu 0x38, DW_OP_shl, DW_OP_constu 0x39, DW_OP_shr, DW_OP_stack_value)
; CHECK: DW_AT_name ("z")
+; CHECK: DW_AT_location (DW_OP_fbreg -4, DW_OP_deref_size 0x8, DW_OP_stack_value)
+; CHECK: DW_AT_name ("q")
define i32 @test4() !dbg !28 {
entry:
@@ -74,6 +76,7 @@ entry:
tail call void @llvm.dbg.declare(metadata ptr %0, metadata !29, metadata !DIExpression(DW_OP_LLVM_extract_bits_zext, 31, 1)), !dbg !30
tail call void @llvm.dbg.declare(metadata ptr %0, metadata !31, metadata !DIExpression(DW_OP_LLVM_extract_bits_sext, 1, 31)), !dbg !30
tail call void @llvm.dbg.declare(metadata ptr %0, metadata !32, metadata !DIExpression(DW_OP_plus_uconst, 3, DW_OP_LLVM_extract_bits_zext, 1, 7)), !dbg !30
+ tail call void @llvm.dbg.declare(metadata ptr %0, metadata !33, metadata !DIExpression(DW_OP_LLVM_extract_bits_zext, 0, 64)), !dbg !30
ret i32 0, !dbg !30
}
@@ -116,3 +119,5 @@ declare void @llvm.dbg.value(metadata, metadata, metadata)
!30 = !DILocation(line: 0, scope: !28)
!31 = !DILocalVariable(name: "y", scope: !28, file: !3, type: !19)
!32 = !DILocalVariable(name: "z", scope: !28, file: !3, type: !9)
+!33 = !DILocalVariable(name: "q", scope: !28, file: !3, type: !34)
+!34 = !DIBasicType(name: "uint64_t", size: 64, encoding: DW_ATE_unsigned)
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