[llvm] 889db04 - Minor optimizations to DW_OP_LLVM_extract_bits_* (#163812)
via llvm-commits
llvm-commits at lists.llvm.org
Tue Oct 21 03:27:58 PDT 2025
Author: Tom Tromey
Date: 2025-10-21T11:27:54+01:00
New Revision: 889db04e2c3b1a4cbcdcc94b7c3e5b365fa2effd
URL: https://github.com/llvm/llvm-project/commit/889db04e2c3b1a4cbcdcc94b7c3e5b365fa2effd
DIFF: https://github.com/llvm/llvm-project/commit/889db04e2c3b1a4cbcdcc94b7c3e5b365fa2effd.diff
LOG: Minor optimizations to DW_OP_LLVM_extract_bits_* (#163812)
I noticed a couple more small optimization opportunities when generating
DWARF expressions from the internal
DW_OP_LLVM_extract_bits_* operations:
* DW_OP_deref can be used, rather than DW_OP_deref_size, when the deref
size is the word size.
* If the bit offset is 0 and an unsigned extraction is desired, then
sometimes the shifting can be skipped entirely, or replaced with
DW_OP_and.
Added:
Modified:
llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
llvm/test/DebugInfo/X86/DW_OP_LLVM_extract_bits.ll
Removed:
################################################################################
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
index f0f086184656a..c7d45897c403b 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
@@ -566,32 +566,54 @@ bool DwarfExpression::addExpression(
case dwarf::DW_OP_LLVM_extract_bits_zext: {
unsigned SizeInBits = Op->getArg(1);
unsigned BitOffset = Op->getArg(0);
+ unsigned DerefSize = 0;
+ // Operations are done in the DWARF "generic type" whose size
+ // is the size of a pointer.
+ unsigned PtrSizeInBytes = CU.getAsmPrinter()->MAI->getCodePointerSize();
// If we have a memory location then dereference to get the value, though
// we have to make sure we don't dereference any bytes past the end of the
// object.
if (isMemoryLocation()) {
- emitOp(dwarf::DW_OP_deref_size);
- emitUnsigned(alignTo(BitOffset + SizeInBits, 8) / 8);
+ DerefSize = alignTo(BitOffset + SizeInBits, 8) / 8;
+ if (DerefSize == PtrSizeInBytes) {
+ emitOp(dwarf::DW_OP_deref);
+ } else {
+ emitOp(dwarf::DW_OP_deref_size);
+ emitUnsigned(DerefSize);
+ }
}
- // Extract the bits by a shift left (to shift out the bits after what we
- // want to extract) followed by shift right (to shift the bits to position
- // 0 and also sign/zero extend). These operations are done in the DWARF
- // "generic type" whose size is the size of a pointer.
- unsigned PtrSizeInBytes = CU.getAsmPrinter()->MAI->getCodePointerSize();
- unsigned LeftShift = PtrSizeInBytes * 8 - (SizeInBits + BitOffset);
- unsigned RightShift = LeftShift + BitOffset;
- if (LeftShift) {
- emitOp(dwarf::DW_OP_constu);
- emitUnsigned(LeftShift);
- emitOp(dwarf::DW_OP_shl);
- }
- 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);
+ // If a dereference was emitted for an unsigned value, and
+ // there's no bit offset, then a bit of optimization is
+ // possible.
+ if (OpNum == dwarf::DW_OP_LLVM_extract_bits_zext && BitOffset == 0) {
+ if (8 * DerefSize == SizeInBits) {
+ // The correct value is already on the stack.
+ } else {
+ // No need to shift, we can just mask off the desired bits.
+ emitOp(dwarf::DW_OP_constu);
+ emitUnsigned((1u << SizeInBits) - 1);
+ emitOp(dwarf::DW_OP_and);
+ }
+ } else {
+ // Extract the bits by a shift left (to shift out the bits after what we
+ // want to extract) followed by shift right (to shift the bits to
+ // position 0 and also sign/zero extend).
+ unsigned LeftShift = PtrSizeInBytes * 8 - (SizeInBits + BitOffset);
+ unsigned RightShift = LeftShift + BitOffset;
+ if (LeftShift) {
+ emitOp(dwarf::DW_OP_constu);
+ emitUnsigned(LeftShift);
+ emitOp(dwarf::DW_OP_shl);
+ }
+ 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
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 5928127866f7e..b79c84d532786 100644
--- a/llvm/test/DebugInfo/X86/DW_OP_LLVM_extract_bits.ll
+++ b/llvm/test/DebugInfo/X86/DW_OP_LLVM_extract_bits.ll
@@ -8,7 +8,7 @@
; CHECK-LABEL: DW_TAG_subprogram
; CHECK: DW_AT_name ("test1")
; CHECK: DW_TAG_variable
-; CHECK: DW_AT_location (DW_OP_fbreg -1, DW_OP_deref_size 0x1, DW_OP_constu 0x3d, DW_OP_shl, DW_OP_constu 0x3d, DW_OP_shr, DW_OP_stack_value)
+; CHECK: DW_AT_location (DW_OP_fbreg -1, DW_OP_deref_size 0x1, DW_OP_constu 0x7, DW_OP_and, DW_OP_stack_value)
; CHECK: DW_AT_name ("x")
; CHECK: DW_TAG_variable
; CHECK: DW_AT_location (DW_OP_fbreg -1, DW_OP_deref_size 0x1, DW_OP_constu 0x39, DW_OP_shl, DW_OP_constu 0x3c, DW_OP_shra, DW_OP_stack_value)
@@ -25,7 +25,7 @@ entry:
; CHECK-LABEL: DW_TAG_subprogram
; CHECK: DW_AT_name ("test2")
; CHECK: DW_TAG_variable
-; CHECK: DW_AT_location (DW_OP_breg0 {{R[^+]+}}+0, DW_OP_constu 0xff, DW_OP_and, DW_OP_constu 0x3d, DW_OP_shl, DW_OP_constu 0x3d, DW_OP_shr, DW_OP_stack_value)
+; CHECK: DW_AT_location (DW_OP_breg0 {{R[^+]+}}+0, DW_OP_constu 0xff, DW_OP_and, DW_OP_constu 0x7, DW_OP_and, DW_OP_stack_value)
; CHECK: DW_AT_name ("x")
; CHECK: DW_TAG_variable
; CHECK: DW_AT_location (DW_OP_breg0 {{R[^+]+}}+0, DW_OP_constu 0xff, DW_OP_and, DW_OP_constu 0x39, DW_OP_shl, DW_OP_constu 0x3c, DW_OP_shra, DW_OP_stack_value)
@@ -67,7 +67,7 @@ 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_location (DW_OP_fbreg -4, DW_OP_deref, DW_OP_stack_value)
; CHECK: DW_AT_name ("q")
define i32 @test4() !dbg !28 {
More information about the llvm-commits
mailing list